Fulfil mission with coating (4)
'coating' is the definition.
(I have seen 'Thin coating' mean 'film' so perhaps 'coating' could also mean 'film')
'fulfil mission with' is the wordplay.
'with' indicates the answer is hidden within the clue.
'FILM' is hidden within 'fulfil mission'.
(Other definitions for film that I've seen before include "Screen", "Thin membrane", "An entertainment; thin layer", "Thin covering or photographic medium (4)", "Material for wrapping".)