There's trouble when he embraces the wrong lass (6)
I believe the answer is:
hassle
'there's trouble when he embraces the wrong lass' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'he embraces the wrong lass' is the wordplay.
'embraces' indicates putting letters inside.
'the wrong' indicates an anagram.
'lass' is an anagram of 'assl'.
'he' going around 'assl' is 'HASSLE'.
Can you help me to learn more?
(Other definitions for hassle that I've seen before include "argument" , "Difficulty - dispute" , "Informal annoyance or quarrel" , "Informally annoy or bother" , "Lashes (anag)" .)