[BluffTHIS!]

The field is definintely too large to lead into with an A on the flop, because lots of players with an A and 2 or 3 broadways will call and when a K or Q or even any other low non-straigtening card comes you will then be beat and not know it, in addition to drawing weak. So I would check and fold. You could call if you were getting 3 plus callers and then take off a card hoping to spike the nut 6. But even then, if you hit that and lead the turn you usually will just get called and will never know where you stand when a scare card comes on the river. And if you check with the intention of raising, you will often just be giving a free card. Plus there is the possibility of someone making the same straight with you but be freerolling to a better one. This just isn't a hand you want to risk your stack on out of position with a large crowd seeing the flop on that board.

Regarding Tilt's comments about raising this preflop from the SB, this play is made a lot at the higher levels where deep stacks are involved and the SB might minraise, not with the intention of representing the A in a large field (although headsup you would), but with the intention of getting reraised by AA/KK and generating a big pot and getting someone with such a hand to dump their stack on a good flop for the SB's hand.