Reserve ship supporting energy weapon (7)
I believe the answer is:
earmark
'reserve' is the definition.
(I know that reserve can be written as earmark)
'ship supporting energy weapon' is the wordplay.
'ship' becomes 'ark' (as in Noah's ark).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'energy' becomes 'e' (physics symbol as in E=mcr).
'weapon' becomes 'arm' (synonyms).
'e'+'arm'='earm'
'ark' put after 'earm' is 'EARMARK'.
(Other definitions for earmark that I've seen before include "Set aside for later" , "reserve for future use" , "'Reserve, set aside (7)'" , "Reserve for later use" , "spot where The Ring may be put on?" .)