Expert's popular photograph (7)
I believe the answer is:
hotshot
'expert's' is the definition.
(hotshot is a kind of expert)
'popular photograph' is the wordplay.
'popular' becomes 'hot' ('hot' can be similar in meaning to 'popular').
'photograph' becomes 'shot' (shot is a kind of photograph).
'hot'+'shot'='HOTSHOT'
(Other definitions for hotshot that I've seen before include "Showily successful person" , "ace" , "Important person" , "Top player" , "Expert" .)