Reserve electronic weapon on ship (7)
I believe the answer is:
earmark
'reserve' is the definition.
(synonyms)
'electronic weapon on ship' is the wordplay.
'electronic' becomes 'e' (as in e-mail or e-books).
'weapon' becomes 'arm' (synonyms).
'on' is a charade indicator (letters next to each other).
'ship' becomes 'ark' (as in Noah's ark).
'e'+'arm'+'ark'='EARMARK'
(Other definitions for earmark that I've seen before include "Set aside for a specific purpose" , "Set aside for future use" , "It identifies the outcome" , "Set aside specifically" , "Set aside for later" .)