In refrain, trombone leads to start of music (5)
I believe the answer is:
intro
'start of music' is the definition.
(short for introduction)
'in refrain trombone leads' is the wordplay.
'in' indicates the answer is hidden within the clue.
'INTRO' can be found hidden inside 'refrain trombone leads'.
'to' acts as a link.
(Other definitions for intro that I've seen before include "portion of song" , "Beginning passage" , "First part of a news story (abbr)" , "Opener" , "Opening bars to a piece of popular music" .)