Missile thrown back with energy in exchange (5-2)
I believe the answer is:
trade-in
'exchange' is the definition.
(I know that trade in is a more specific form of exchange)
'missile thrown back with energy in' is the wordplay.
'missile' becomes 'dart' (Backwards).
'thrown back' is a reversal indicator.
'with' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'dart' reversed gives 'trad'.
'trad'+'e'+'in'='TRADE-IN'
(Other definitions for trade-in that I've seen before include "Old item given in part payment for a new one" , "Older article used in part payment to retailer" , "Article used in part-exchange" , "Article used in part payment for another" , "Something used in part exchange" .)