Hold back substitute (7)
I believe the answer is:
reserve
I believe this is a double definition.
'hold back' is the first definition.
(I've seen this before)
'substitute' is the second definition.
(synonyms)
(Other definitions for reserve that I've seen before include "not in the front line" , "Book - substitute player" , "keep for oneself" , "cool" , "Modesty- store" .)