It really depends...
That is probably some sort of feeler bet. With a hand like
![The Ace of Clubs [Ac]](https://www.pofex.com/images/smilies/Ac.gif)
. I would probably just call, trying to see a cheap turn card to try and fill. If you don't fill on the turn and he bets at it big again, you're probably beat. But he should ask himself, would he raise with TT in sb like that? Would he raise with AQd or AKd and lead out on the flop?
But on the surface it does look like it's a big pair trying to protect himself, and may also have the diamond, but maybe not. I would probably just call on this flop, but the other problem is the other guy in the pot. If he raises, then you're a gone goose.
Very tricky hand to play, how did it turn out?