A possibly slow film (3,5)
I believe the answer is:
key largo
'film' is the definition.
(I've seen this before)
'a possibly slow' is the wordplay.
'a possibly' becomes 'key' (I am not sure about this - if you are sure you should believe this answer much more).
'slow' becomes 'largo' (musical term).
'key'+'largo'='KEY LARGO'
Can you help me to learn more?
(Other definitions for key largo that I've seen before include "film" , "island" , "parts here for 1a and 9 [BACALL & BOGART] - with 12a" .)