Just defeat queen’s musician (5)
I believe the answer is:
piper
'musician' is the definition.
(piper is a kind of musician)
'just defeat queen's' is the wordplay.
'just defeat' becomes 'pip' (pipping is a kind of defeating. I am not sure about the 'just' bit.**).
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'pip'+'er'='PIPER'
Can you help me to learn more?
(Other definitions for piper that I've seen before include "Military musician" , "Highland musician" , "Bagpipes player" , "'Liam O'Flynn or Seamus Ennis, say (5)'" , "Musician (paid for a tune?)" .)