Enthusiastic about penning start of refrain -- the first notes (5)
I believe the answer is:
intro
'the first notes' is the definition.
(I've seen this before)
'enthusiastic about penning start of refrain' is the wordplay.
'enthusiastic about' becomes 'into' (I've seen this before).
'penning' means one lot of letters goes inside another.
'start of' says to take the initial letters.
The initial letter of 'refrain' is 'r'.
'into' enclosing 'r' is 'INTRO'.
(Other definitions for intro that I've seen before include "Prologue, opening (abbr.)" , "this may get one started" , "First bars of a piece of popular music" , "Launch" , "Early music piece" .)