Additional drink council leader knocks back on returning (6)
I believe the answer is:
chaser
'additional drink' is the definition.
(alcoholic drink taken with another one)
'council leader knocks back on returning' is the wordplay.
'leader' suggests taking the first letters.
'knocks back' becomes 'has' (to knock back can mean to consume or have).
'on' becomes 're' ('on' can be mean 'with reference to').
'returning' shows that the letters should be reversed in order.
The initial letter of 'council' is 'c'.
're' reversed gives 'er'.
'c'+'has'+'er'='CHASER'
(Other definitions for chaser that I've seen before include "One who pursues - one drink after another" , "Beer after whisky - follow that!" , "Racing horse" , "second drink" , "Racehorse - search (anag)" .)