Keen on including right opening for piece of music (5)
I believe the answer is:
intro
'for piece of music' is the definition.
(intro is a kind of piece of music)
'keen on including right opening' is the wordplay.
'keen on' becomes 'into' (if you're 'into' something you're keen on it).
'including' is an insertion indicator.
'opening' says to take the initial letters.
The initial letter of 'right' is 'r'.
'into' placed around 'r' is 'INTRO'.
(Other definitions for intro that I've seen before include "Opening notes" , "Beginning (colloq.)" , "Prologue, opening (abbr.)" , "Opener" , "Preliminary section (of music) (colloq.)" .)