Set aside weapon in English rescue boat (7)
I believe the answer is:
earmark
'set aside' is the definition.
(synonyms)
'weapon in english rescue boat' is the wordplay.
'weapon' becomes 'arm' (synonyms).
'in' indicates putting letters inside.
'english' becomes 'E' (abbreviation).
'rescue boat' becomes 'ark' (rescue boat during the biblical flood).
'e'+'ark'='eark'
'arm' placed inside 'eark' is 'EARMARK'.
(Other definitions for earmark that I've seen before include "Set aside for future use" , "Allocate for a purpose" , "Designate funds for a particular purpose" , "Set aside for later" , "Set specially aside" .)