You win the the A§ and return the 4 your 4th best and declarer wins the K partner following upwards. He draws trumps in 2 rounds with the A & Q and then leads a diamond to the Q and your K. It's your go again. You might just adopt a passive defense and return the10.Well partner will not be pleased, declarer wins the J and discards a losing heart on the A. As your partner remonstrates with you, you protest how was I to know to lead away from my K. Well let's count declarers tricks. He has 6 trump tricks,1 diamond and 1club totalling 8,what's the club situation?Partner led the 3 and followed upwards therefore declarer has another club which he can ruff bringing his total to 9 tricks so if he has the A   he has 10 tricks, just count.