Bring up film — the girl’s a nonentity (6)
I believe the answer is:
cipher
'a nonentity' is the definition.
('cipher' can be a synonym of 'nonentity')
'bring up film the girl's' is the wordplay.
'bring up' is a reversal indicator.
'film' becomes 'pic' (short for picture).
'the girl' becomes 'her' ('her' refers to a girl or woman).
'pic' back-to-front is 'cip'.
'cip'+'her'='CIPHER'
(Other definitions for cipher that I've seen before include "Secret code" , "Nonentity" , "A message written in code" , "Key to a code" , "nobody" .)