Part of the leg is theme for poet (7)
I believe the answer is:
elegist
'poet' is the definition.
(elegist is a kind of poet)
'part of the leg is theme' is the wordplay.
'part of' indicates a hidden word.
'ELEGIST' is hidden within 'the leg is theme'.
'for' is the link.
(Other definitions for elegist that I've seen before include "one wrote mournfully" , "Composer of mournful songs" , "Gloomy poet" , "serious linesman" , "bemoaning linesman" .)