Concerning, drinking unknown whiskey (3)
I believe the answer is:
rye
'whiskey' is the definition.
(rye whiskey is a type of whiskey)
'concerning drinking unknown' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'drinking' indicates putting letters inside (inserted letters are consumed or drunk).
'unknown' becomes 'y' (algebra).
're' enclosing 'y' is 'RYE'.
(Other definitions for rye that I've seen before include "old port" , "Bitterly regret" , "Whisky; Sussex town" , "Type of whisky" , "Whiskey ingredient" .)