Lee proved to be sporting photographer's solution (9)
I believe the answer is:
developer
'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.
'lee'+'proved'='leeproved'
'leeproved' with letters rearranged gives 'DEVELOPER'.
(Other definitions for developer that I've seen before include "One benefiting from property" , "Property magnate" , "Photographic reagent" , "Film-treating fluid" , "kind of entrepreneur" .)