Fulfil mission with coating (4)
I believe the answer is:
film
'coating' is the definition.
(I've seen this in another clue)
'fulfil mission with' is the wordplay.
'with' indicates a hidden word.
'FILM' can be found hidden inside 'fulfil mission'.
(Other definitions for film that I've seen before include "Flick" , "Thin coating for movie." , "reels, perhaps" , "Screen" , "Thin layer (covering a surface)" .)