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' says to put 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'" .)