Perhaps Ulster's best film (7)
I believe the answer is:
topcoat
'perhaps ulster's' is the definition.
(an Ulster is a type of coat)
'best film' is the wordplay.
'best' becomes 'top' ('top' can be similar in meaning to 'best').
'film' becomes 'coat' (I've seen this in other clues).
'top'+'coat'='TOPCOAT'
(Other definitions for topcoat that I've seen before include "Final layer of paint" , "Ulster?" , "It's worn on the outside in winter" , "Heavy winter outer garment" , "clothing" .)