The bidding proceeded
East South West North
Pass 1♦ Pass 3♦
3♥ 4NT Pass 5♦
Pass Pass Pass
This was what West saw after leading
East won the A and returned a heart to South's Q. South led ♦Q, covered by the K and taken with the A, East following. Declarer drew the last trump with dummy's 9, then led the ♣9 to the K and West's A. What should West do now?
Well, West is in a pickle, down to spades and clubs. Either one is likely to give away the game-going trick. West in fact led a spade, and when dummy's J held, the hand was over. The full deal:
West should return a club after winning the A (anything but the Q). South surely has both spade honors for his 4NT bid, and has no reason not to play the J. That play loses only if East has the bare ♠Q or Qx and the ♣Q cannot be ruffed out. By contrast, on the lead of a low club, South might play to ruff out the ♣Q or drop the ♠Q. If he plays a spade from dummy instead, he loses immediately if East has the ♣Q. While it's still probably best to play West for the ♣Q, at least South has a chance to go wrong.
Also note that South can improve the play slightly by cashing the ♠A before playing on trumps.
A word about the bidding:
I was sitting South. I thought partner's bid was standard, since we hadn't agreed otherwise. Of course, he'd already done some overbidding
Edited to say: By the way, declarer might also think of a black suit squeeze on West, but it's not happening. If he tries unblocking the spade honors, he has no entry left (after ruffing 2 clubs and a heart and cashing the high trump from hand) to the club J. If he keeps the entry, then his hand is squeezed before West.

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