Join the army again? One is not in the front line (7)
I believe the answer is:
reserve
I believe this is a double definition.
'join the army again?' is the first definition.
(serve with the army again)
'one is not in the front line' is the second definition.
(army reserves are not on the front line)
(Other definitions for reserve that I've seen before include "Modesty- store" , "cool" , "One on bench" , "Kept back stock" , "ice" .)