Fiend barely cased behind bar, returning for painkiller (9)
I believe the answer is:
ibuprofen
'for painkiller' is the definition.
(ibuprofen is a kind of painkiller)
'fiend barely cased behind bar returning' is the wordplay.
I cannot really see how this works, but
'behind' could be 'pro' (to be pro something is to be behind it) and 'pro' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ibuprofen that I've seen before include "Painkiller" , "anaesthetic" , "Anti-inflammatory medicine" , "Drug" .)