Concerning ex-Liberal (8)
I believe the answer is:
reformer
'liberal' is the definition.
(thesaurus)
'concerning ex' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'ex' becomes 'former' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'former'='REFORMER'
Can you help me to learn more?
(Other definitions for reformer that I've seen before include "eg Luther" , "Person who advocates change" , "One making improving changes" , "One dealing with abuses" , "One changing for the better" .)