Fierce woman knocking alcohol back — concerning (6)
I believe the answer is:
gorgon
'fierce woman' is the definition.
(monster in Greek myth)
'knocking alcohol back concerning' is the wordplay.
'knocking' says the letters should be written in reverse.
'alcohol' becomes 'grog' (grog is a kind of alcohol).
'back concerning' becomes 'on' (I am not sure about the 'back' bit.).
'grog' backwards is 'gorg'.
'gorg'+'on'='GORGON'
(Other definitions for gorgon that I've seen before include "In Greek myth, her glance turned people to stone" , "Mythical female winged monster" , "Snake-haired monster" , "Mythical sister with snakes for hair" , "Medusa was one" .)