Concerned with sight, squint finally through old film (5)
I believe the answer is:
optic
'concerned with sight' is the definition.
(optic means relating to the eye)
'squint finally through old film' is the wordplay.
'finally' suggests the final letters.
'through' is an insertion indicator (inserted letters go through the word).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'film' becomes 'pic' (short for picture).
The last letter of 'squint' is 't'.
'o'+'pic'='opic'
't' inserted within 'opic' is 'OPTIC'.
(Other definitions for optic that I've seen before include "Relating to the eyes" , "Device for measuring dispensed alcohol" , "Pertaining to sight" , "One sees" , "Spirit dispenser - of the eye" .)