One sportsman to be all for reform (10)
I believe the answer is:
footballer
'one sportsman' is the definition.
(I know that footballer is a type of sportsman)
'to be all for reform' is the wordplay.
'reform' indicates an anagram (letters in a new form).
'to'+'be'+'all'+'for'='tobeallfor'
'tobeallfor' is an anagram of 'FOOTBALLER'.
(Other definitions for footballer that I've seen before include "left-winger, perhaps" , "Back (or forward?)" , "Sportsman" , "Kenny Dalglish?" .)