Reserve weapon carried by English vessel (7)
I believe the answer is:
earmark
'reserve' is the definition.
(synonyms)
'weapon carried by english vessel' is the wordplay.
'weapon' becomes 'arm' (synonyms).
'carried by' indicates putting letters inside.
'english' becomes 'E' (abbreviation).
'vessel' becomes 'ark' (ark is a kind of vessel).
'e'+'ark'='eark'
'arm' going within 'eark' is 'EARMARK'.
(Other definitions for earmark that I've seen before include "It identifies the outcome" , "Designate funds for a particular purpose" , "Set aside for later" , "Designate for a specific purpose" , "'Reserve, set aside (7)'" .)