Keen on adopting Rachmaninoff’s first prelude (5)
I believe the answer is:
intro
'prelude' is the definition.
(I've seen this before)
'keen on adopting rachmaninoff's first' is the wordplay.
'keen on' becomes 'into' (I've seen this in another clue).
'adopting' indicates putting letters inside (inserted letters are taken on or adopted).
'rachmaninoff's first' becomes 'r' (1st letter of 'rachmaninoff').
'into' enclosing 'r' is 'INTRO'.
(Other definitions for intro that I've seen before include "(Short) section before Chapter 1, say" , "Launch" , "Jazz piece" , "starter" , "Opening notes" .)