When you say cutoff 3bet 6k i guess you meant he bet 6k.
At that point you have 13bb you could push which would increase your stack of about 23% if he folds. If his stack is between 26k-46k your fold equity is pretty high.
Against an unknown player it seems to me the best play for you is to push and don't see a flop.
Playing post flop would be hard for you to play, if you missed the flop what would you do ? since the pot is already 14.600 and you only have about 20k left. You are first to act there aren't many possibilities here(I will try to explain the 2 situations when you hit the flop and if you had missed the flop).
1.Possibility (I would recommend this if you called PF bet)
You push on flop whatever comes. (Risky but since he most likely missed the flop you have great chances to win the pot) People only hit 20% of flops. Most of the time when people raise higher than normaly it means his cards are high cards like QJ+. But it realy depends on the player it differs from one to another some have 56s hands when raising more. This way you can easyer found out what flop you can push or not. The key to make better decision is to know his betting pattern. So since i would think he hit that flop but i don't want to risk him to get a free card i think all in is best decision since if he hits he calls your all in and if he missed he folds and doesn't get his free card and you move up 23% of your initial stack.
2.Possibility (There's no way this could work unless you are playing against scary player)
-You raise some amount like 3bb and the other guy folds. (I don't think this could work because he's the chip leader)
-If he doesn't fold you could still check fold turn and survive with 7 bb if you didn't hit flop/turn.
-Since you hit flop if it's not a scary card(T,J,Q) you could check raise all in or check turn, bet allin on river.
Check and see what he will do. Depending on the player you will have 3 answers.
-He pushes. Even if you didn't hit the flop i wouldn't fold instantly, you could still be ahead(which is why its better to take the initiative to push to have some fold equity).
Since he's chip leader he could have tried a move here with air(nothing hit on flop).
-But since you hit the flop it is an easy call. I don't see how you could fold here unless you have a monster read on him that he had KQ.(Easiest way to know this is his betting pattern like Does he go only all in with good hands etc.. and if his 3bb PF meant good hand because normaly 2bb is standard)
-He raises some amount <4BB. You could push or fold. Since you don't know him i would push.(At that point your fold equity is pretty nonexistent and it would seem like he's trying to steal but doesn't want to commit your whole stack.)