Bird one's put in film (6)
I believe the answer is:
siskin
'bird' is the definition.
(siskin is a kind of bird)
'one's put in film' is the wordplay.
'one's' becomes 'is' (roman numeral).
'put in' indicates putting letters inside.
'film' becomes 'skin' (I've seen this in other clues**).
'is' placed into 'skin' is 'SISKIN'.
(Other definitions for siskin that I've seen before include "Type of finch" , "Small bird - kiss in (anag)" , "Yellow-and-black finch" , "Little bird" , "Small yellow and black finch" .)