Soldiers caught blackleg for nothing (6)
I believe the answer is:
gratis
'nothing' is the definition.
(I've seen this before)
'soldiers caught blackleg' is the wordplay.
'soldiers' becomes 'gis' (I've seen this in other clues).
'caught' is an insertion indicator.
'blackleg' becomes 'rat' ('rat' can be a synonym of 'blackleg').
'gis' placed around 'rat' is 'GRATIS'.
'for' is the link.
(Other definitions for gratis that I've seen before include "Free, at no cost" , "Free (Latin)" , "Without charge" , "Free, costs nothing" , "Costing nothing, free" .)