Drink more rum, boss (7)
I believe the answer is:
supremo
'boss' is the definition.
(I've seen this before)
'drink more rum' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'rum' indicates anagramming the letters (rum can mean strange or unusual).
'more' with letters rearranged gives 'remo'.
'sup'+'remo'='SUPREMO'
(Other definitions for supremo that I've seen before include "big Daddy" , "Overall chief" , "One in overall charge" , "Person in overall authority" , "Top dog" .)