Characters from Kew longed to develop expertise (9)
I believe the answer is:
knowledge
'expertise' is the definition.
(thesaurus)
'characters from kew longed to develop' is the wordplay.
'kew longed' anagrams to 'KNOWLEDGE'.
However, I'm not sure how this is indicated.
(Other definitions for knowledge that I've seen before include "Result of learning - awareness" , "Information - erudition" , "Information; awareness" , "a small amount is risky" , "A little of it is said to be a dangerous thing" .)