Loose and weak heavyweight was victorious over soldier (6)
I believe the answer is:
wanton
'loose' is the definition.
(I know that loose can be written as wanton)
'was victorious over soldier' is the wordplay.
'was victorious' becomes 'won' (I've seen this in another clue).
'over' is an insertion indicator.
'soldier' becomes 'ant' (soldier is a type of ant).
'won' going around 'ant' is 'WANTON'.
'and weak heavyweight' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for wanton that I've seen before include "Thoughtlessly and unnecessarily cruel" , "Dissolute person" , "Dissolute and shameless" , "(Of violence) deliberate and unprovoked" , "Thoughtlessly cruel or motiveless" .)