Rescue brute that's left inside (7)
I believe the answer is:
salvage
'rescue' is the definition.
(I know that salvage is a type of rescue)
'brute that's left inside' is the wordplay.
'brute' becomes 'savage' (synonyms).
'that's left' becomes 'l' (common abbreviation).
'inside' means one lot of letters goes inside another.
'savage' enclosing 'l' is 'SALVAGE'.
(Other definitions for salvage that I've seen before include "Rescue (and recycle?)" , "get back" , "Rescue of a damaged ship or its cargo" , "Rescue of a ship's cargo" , "Rescue of goods from peril" .)