All three suits have been bid during the auction so best to lead the unbid suit "and TRY & FIND PARTNER"

Trick 1 is won by declarer's ♦9.

Declarer now plays a Club to dummy's Queen and a club back to the King. You win this third trick and play another Diamond won by dummy's Ten. Declarer now plays the Spade Queen from dummy letting it run and you win your king and try a low Heart. Declarer inserts the Ten thinking its safe to let East win . However East wins and now cashes 3 Diamond tricks to defeat the contract by two. The defence made 1 Spade trick, 1 Club trick, 1 Heart trick and eventually 3 Diamond tricks. This hand just shows you to persevere and your partner's suit will be well and truly established for  the defence. (NB Partner was on same wavelength and didn't give the impression that he had the Diamond suit)