After both you and partner bid Diamonds partner leads a Club.

When declarer follows suit at trick 1 with the ♣2 you  can now  deduce that partner has led a singleton.

You will need to play a club back with a specific message where your entry lies so you can give partner a second ruff.

At trick 2 play back your lowest club (♣6) as a suit preference for Diamonds telling partner that's where your entry is.

As expected, partner duly ruffs the second trick and with a singleton Diamond showing in dummy now plays a low Diamond (not his Ace) and you are able to win the third trick and give partner a second ruff.

You have now taken the contract one off.

Without this defence declarer will be able to quickly draw trumps and establish his club suit.

(NB Even if you do not know about suit preference signals West might try underleading his Diamond after receiving the first ruff as he can see the 2 Aces in dummy and that would be his only choice of you being able to get the lead PLUS you supported Diamonds during the auction).