I threw this into Bridge Winners and an early response came from Michael Rosenberg (I inserted some notes, largely because I made mistakes in my post, using the following format <text - JC>):
"First, the dummy has 12 cards. Presumably the 'compensating' error of having dummy play, at trick one, a card not in dummy solves that problem. <my fault - I left out the
2 - JC>
"Is declarer really missing the ♥ K? This seems unlikely on the auction, whatever that 4NT meant."
Dummy bid 4N.
"Declarer bid 4NT holding five plus hearts to the J." <leftover problem from Jack's mis-entering the auction and my using Ken's statement verbatim before the error was pointed out - JC>
Declarer did not bid 4N
Our defensive carding is not given (which is not good). I'm assuming 'right side up'.
Apart from all that, the OP was ok.
* * * * *
Declarer's hand is QJ10, Kxxxxx, AK, 8x
I must return a spade, or partner gets squeezed in the majors. Partner should probably have seen this coming and risen with the ♠K from K975. This would have been more attractive with K1075 - which is why I gave declarer ♠QJ10 ibstead of QJ9.
Declarer obviously has no ♥J.
Declarer COULD have played for a legitimate make - partner having ♠AK and 3(+)-card ♥. He can't be 'punished' - but should not be 'rewarded'.
* * * * *
The above is the 'obvious' answer. But since it's posted as an option (AND the most 'detailed' option), I'm inclined to believe that declarer had a different hand. If partner has ♦A and no ♦J, he would have played 1dA at trick one and hoped I had ♠A (possibly should do that even with ♦J). So, that means declarer bid 6N with only KQJ, Kxxxxx. Kx, xx - seems most unlikely.
How about partner having ♥K? Declarer might have KQ9, J9xxxx, AK, xx. In this case, declarer can't make - as long as I can, with my carding, persuade partner to hold spades and ditch hearts.
Of course, if declarer started with K-Q-TEN of spades, he will make. And then I already blew the defense by not ducking the spade smoothly. Perhaps that is the:
"Something ele (sic) is going on (please comment to clarify)."
However, since the OP went out of it's way " He did this on a heart suit headed by the J? So let's assume declarer holds the ♥ K."
So I'm stuck with the 'obvious' answer - return a spade. I hope I'm missing something. "