Submarine captain backing old crew members (4)
I believe the answer is:
nemo
'submarine captain' is the definition.
(character from 20,000 Leagues Under the Sea)
'backing old crew members' is the wordplay.
'backing' shows that the letters should be reversed in order.
'old' becomes 'o' (abbreviation).
'crew members' becomes 'men' (I've seen this in another clue).
'o'+'men'='omen'
'omen' written backwards gives 'NEMO'.
(Other definitions for nemo that I've seen before include "Jules Verne's submarine captain" , "Jules Verne captain" , "- me impune lacessit (motto of Scotland)" , "Literary captain" , "commander for the Nautilus" .)