Thump coming across ace's midriff (6)
I believe the answer is:
paunch
'midriff' is the definition.
(thesaurus)
'thump coming across ace's' is the wordplay.
I cannot quite see how this works, but
'ace' could be 'a' (abbreviation) and 'a' is found within the answer.
'thump' could be 'punch' (I've seen this before) and 'punch' is found within the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for paunch that I've seen before include "Fat belly" , "Large stomach or belly" , "Beer belly" , "Beer-belly" , "corporation" .)