Petersham's much-admired film? (9)
I believe the answer is:
greatcoat
'petersham's' is the definition.
(Petersham is a type of greatcoat)
'much-admired film?' is the wordplay.
'much admired' becomes 'great'.
'film?' becomes 'coat' (I've seen this in other clues).
'great'+'coat'='GREATCOAT'
(Other definitions for greatcoat that I've seen before include "what a soldier might wear" , "costume?" , "Long heavy outer garment" , "Military outer wear" , "One may cover officer" .)