Entertainer's beginning with joke outfit (5)
I believe the answer is:
equip
'outfit' is the definition.
(I know that outfit can be written as equip)
'entertainer's beginning with joke' is the wordplay.
'beginning' says to take the initial letters.
'with' means one lot of letters go next to another.
'joke' becomes 'quip' (a quip is a joke or witticism).
The first letter of 'entertainers' is 'e'.
'e'+'quip'='EQUIP'
(Other definitions for equip that I've seen before include "Supply (with items)" , "Sally" , "Supply with what is needed" , "Provide what's needed" , "Outfit" .)