Greed, a weakness holding soldiers back (7)
I believe the answer is:
avarice
'greed' is the definition.
(synonyms)
'a weakness holding soldiers back' is the wordplay.
'weakness' becomes 'vice' (I've seen this in another clue).
'holding' is an insertion indicator.
'soldiers' becomes 'RA' (abbreviation for Royal Artillery).
'back' shows that the letters should be reversed in order.
'ra' backwards is 'ar'.
'vice' enclosing 'ar' is 'varice'.
'a'+'varice'='AVARICE'
(Other definitions for avarice that I've seen before include "Cupidity" , "Greed - caviare (anag)" , "Greed for gain" , "Extreme greed for wealth" , "Excessive greed for wealth" .)