Soldiers watching decide on film (6)
I believe the answer is:
picket
'soldiers watching' is the definition.
(soldiers watching for an enemy)
'decide on film' is the wordplay.
'decide' becomes 'pick' (to pick something is to decide on it).
'on' is a charade indicator (letters next to each other).
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'pick'+'et'='PICKET'
(Other definitions for picket that I've seen before include "Striking protesters" , "Protesting striker" , "Wooden stake, strike group" , "Kind of fence or striking workers" , "Group protecting strikers" .)