Stop former soldier meeting second in command (4)
I believe the answer is:
veto
'stop' is the definition.
(I've seen this before)
'former soldier meeting second in command' is the wordplay.
'former soldier' becomes 'vet' (short for veteran).
'meeting' is a charade indicator (letters next to each other).
'second in command' becomes 'o' (2nd letter of 'command').
'vet'+'o'='VETO'
(Other definitions for veto that I've seen before include "Forbid outright" , "Refuse to authorise" , "Stop" , "The right to forbid an action" , "Literally, 'I forbid'" .)