What's payable for loss-leader causes fight (4)
I believe the answer is:
duel
'fight' is the definition.
(dueling is a kind of fighting)
'what's payable for loss-leader' is the wordplay.
'what's payable' becomes 'due' (similar in meaning).
'for' is a charade indicator (letters next to each other).
'leader' indicates taking the first letters.
The initial letter of 'loss' is 'l'.
'due'+'l'='DUEL'
'causes' acts as a link.
(Other definitions for duel that I've seen before include "Combat to settle a row" , "Prearranged engagement" , "Fight to the death between two people" , "shoot-out?" , "Contest between two with deadly weapons" .)