Keen on captivating start to Rameau prelude (5)
I believe the answer is:
intro
'prelude' is the definition.
(I've seen this before)
'keen on captivating start to rameau' is the wordplay.
'keen on' becomes 'into' (I've seen this before).
'captivating' means one lot of letters goes inside another (I've seen this in other clues).
'start to' indicates taking the first letters.
The first letter of 'rameau' is 'r'.
'into' placed around 'r' is 'INTRO'.
(Other definitions for intro that I've seen before include "First notes" , "Preliminary section (of music) (colloq.)" , "Early music piece" , "Nitro (anag.)" , "Opening notes" .)