Semi-pro developing potential (7)
I believe the answer is:
promise
'potential' is the definition.
(I've seen this in another clue)
'semi-pro developing' is the wordplay.
'developing' indicates anagramming the letters (letters developed into a new form).
'semi'+'pro'='semipro'
'semipro' is an anagram of 'PROMISE'.
(Other definitions for promise that I've seen before include "Give an undertaking" , "Assurance" , "Vow" , "Pledge - my word!" , "Betroth" .)