One entertains British monarch outside Usk (6)
I believe the answer is:
busker
'one entertains' is the definition.
I can't tell whether this definition defines the answer.
'british monarch outside usk' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'monarch' becomes 'er' (abbreviation for Elizabeth Regina, British Queen).
'outside' is an insertion indicator.
'b'+'er'='ber'
'ber' placed around 'usk' is 'BUSKER'.
Can you help me to learn more?
(Other definitions for busker that I've seen before include "Street performer" , "Public entertainer" , "player in street" , "Street entertainer" , "Street musician" .)