Lee proved to be sporting photographer's solution (9)
I believe the answer is:
Here is my best explanation:
'photographer's solution' is the definition.
(chemical used to develop photographs)
'lee proved to be sporting' is the wordplay.
'to be sporting' indicates anagramming the letters.
'leeproved' with letters rearranged gives 'DEVELOPER'.
(Other definitions for developer that I've seen before include "Property magnate", "One benefiting from property", "Photographic reagent", "solution for picture-maker", "It creates image", "that produces positive results", "striving to achieve a positive image?", "Film-treating fluid" and "kind of entrepreneur".)