Military operatives upset over submarine captain (4)
I believe the answer is:
nemo
'submarine captain' is the definition.
(character from 20,000 Leagues Under the Seas)
'military operatives upset over' is the wordplay.
'military operatives' becomes 'men'.
'upset' shows that the letters should be reversed in order.
'over' becomes 'o' (cricket abbreviation).
'men' written backwards gives 'nem'.
'nem'+'o'='NEMO'
(Other definitions for nemo that I've seen before include "- me impune lacessit (motto of Scotland)" , "commander for the Nautilus" , "Literary captain" , "Jules Verne's submarine captain" , "Captain brought to book" .)