Reserve diver (3)
I believe the answer is:
sub
This clue is a double definition.
'reserve' is the first definition.
(substitute)
'diver' is the second definition.
(submarine)
(Other definitions for sub that I've seen before include "Prefix meaning under or inferior" , "Prefix meaning beneath" , "Underwater boat" , "Underwater craft" , "Warship; membership fee" .)