Bar measure, first of three in old photograph (5)
I believe the answer is:
optic
'bar measure' is the definition.
(I've seen this before)
'first of three in old photograph' is the wordplay.
'first of three' becomes 't' (1st letter of 'three').
'in' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'photograph' becomes 'pic' (short for picture).
'o'+'pic'='opic'
't' going into 'opic' is 'OPTIC'.
(Other definitions for optic that I've seen before include "Spirit measuring device" , "Alcoholic drink dispenser" , "Bar measure" , "Relating to the eyes" , "Visual" .)