Honest fellow managed onset of kleptomania (5)
I believe the answer is:
frank
'honest' is the definition.
(similar in meaning)
'fellow managed onset of kleptomania' is the wordplay.
'fellow' becomes 'F' (abbreviation - of a society etc.).
'managed' becomes 'ran' (both can mean organised or was in charge of).
'onset of' indicates taking the first letters.
The initial letter of 'kleptomania' is 'k'.
'f'+'ran'+'k'='FRANK'
(Other definitions for frank that I've seen before include "Candid, honest" , "Outspoken" , "Without evasion" , "Open" , "Blunt; honest" .)