Nice ladies cross over sweetheart in trouble (8)
I believe the answer is:
mesdames
'nice ladies' is the definition.
'mesdames' can be an answer for 'ladies' ('madame' can be a synonym of 'lady'). I'm not sure about the 'nice' bit.
'cross over sweetheart in trouble' is the wordplay.
'cross over sweetheart' becomes 'dame' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'trouble' becomes 'mess'.
'dame' placed within 'mess' is 'MESDAMES'.
Can you help me to learn more?
(Other definitions for mesdames that I've seen before include "Messieurs et ... - French ladies" , "Ladies and gentlemen; ........ et monsieurs" .)