Partners back film (7)
I believe the answer is:
western
'film' is the definition.
(Western is an example)
'partners back' is the wordplay.
'partners' becomes 'we'.
'back' becomes 'stern' (I've seen this before).
'we'+'stern'='WESTERN'
(Other definitions for western that I've seen before include "Cowboy film of occidental origin" , "direction?" , "Old film genre, horse opera" , "American film" , "Type of film or music" .)