Your hold: ♠ A63 ♥ AJ742 ♦ KJ5 ♣ 85

Lead the diamond king (or, if you prefer, the jack). It does not look like declarer has a balanced 25-count. He probably has a running long suit, namely clubs. In that case, you may need to go active in search of tricks. Partner did not bid spades or raise hearts, so try a diamond, specifically a high one to prevent the suit from blocking. It can’t hurt to cross your fingers surreptitiously at the same time.