A bird seen in the music-hall (3)
I believe the answer is:
emu
'a bird' is the definition.
(emu is a type of bird)
'in the music-hall' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EMU' can be found hidden inside 'the music hall'.
'seen' is the link.
(Other definitions for emu that I've seen before include "Ratite" , "Extinct bird" , "Australian non-flyer" , "Native Australian bird" , "Physics unit" .)