Keen on acquiring Rachmaninoff's first prelude (5)
I believe the answer is:
intro
'prelude' is the definition.
(I've seen this before)
'keen on acquiring rachmaninoff's first' is the wordplay.
'keen on' becomes 'into' (I've seen this before).
'acquiring' is an insertion indicator.
'first' suggests taking the first letters.
The first letter of 'rachmaninoff' is 'r'.
'into' enclosing 'r' is 'INTRO'.
(Other definitions for intro that I've seen before include "preface" , "Musical opening" , "Prologue, opening (abbr.)" , "Opening section" , "overture" .)