Defeatist never gives the positive side of the picture (8)
I believe the answer is:
negative
'defeatist' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for negative that I've seen before include "The photographic film expresses refusal" , "Less than zero quantity" , "A test result" , "Get a vine (anag.)" , "image for a developer" .)