Absolutely certain it's a photographic term (8)
I believe the answer is:
positive
I believe this is a double definition.
'absolutely certain' is the first definition.
(I've seen this in another clue)
'a photographic term' is the second definition.
(as opposed to a photographic negative)
'it's' acts as a link.
(Other definitions for positive that I've seen before include "Optimistic; certain" , "Confident, very sure" , "Definite, sure" , "Definite, confident" , "Of a number, greater than zero - that's certain" .)