Drink on tick for boss (7)
I believe the answer is:
supremo
'boss' is the definition.
(I've seen this before)
'drink on tick' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'on' becomes 're' ('on' can be mean 'with reference to').
'tick' becomes 'mo' (both can mean a very short amount of time).
'sup'+'re'+'mo'='SUPREMO'
'for' acts as a link.
(Other definitions for supremo that I've seen before include "Top commander" , "big Daddy" , "top boss" , "Champion" , "Top dog" .)