Leader finished with band that's very entertaining (7)
I believe the answer is:
supremo
'leader' is the definition.
(I've seen this before)
'finished with band that's very entertaining' is the wordplay.
'finished' becomes 'up' (eg 'time's up' means the time is finished).
'with' says to put letters next to each other.
'band' becomes 'rem' (REM are a US rock band).
'that's very' becomes 'so' (both can mean 'extremely').
'entertaining' is an insertion indicator.
'up'+'rem'='uprem'
'uprem' placed within 'so' is 'SUPREMO'.
(Other definitions for supremo that I've seen before include "Overall head" , "Top commander" , "top boss" , "C-in-C" , "Person in overall charge, big boss" .)