Boxes, mostly thin on the ground (5)
I believe the answer is:
spars
'boxes' is the definition.
(sparring is a kind of boxing)
'mostly thin on the ground' is the wordplay.
'mostly' means to remove the last letter.
'thin on the ground' becomes 'sparse' (I've seen this before).
'sparse' with its last letter taken off is 'SPARS'.
(Other definitions for spars that I've seen before include "Boxes" , "disputes" .)