Concerning one-time worker for improvement (8)
I believe the answer is:
reformer
'worker for improvement' is the definition.
I can't tell whether this defines the answer.
'concerning one-time' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'one-time' becomes 'former' (I've seen this in another clue).
're'+'former'='REFORMER'
Can you help me to learn more?
(Other definitions for reformer that I've seen before include "eg Luther" , "One making improving changes" , "One changing for the better" , "campaigner for change" , "One who changes or improves, say, an organisation" .)