Greed - a sin besetting returning soldiers (7)
I believe the answer is:
avarice
'greed' is the definition.
(avarice is a kind of greed)
'a sin besetting returning soldiers' is the wordplay.
'sin' becomes 'vice'.
'besetting' means one lot of letters goes inside another.
'returning' says the letters should be written in reverse.
'soldiers' becomes 'ra' (abbreviation for Royal Artillery).
'ra' reversed gives 'ar'.
'vice' going around 'ar' is 'varice'.
'a'+'varice'='AVARICE'
(Other definitions for avarice that I've seen before include "Greed for gain" , "Stinginess" , "Extreme greed for wealth" , "Rapacity" , "Excessive greed for wealth" .)