Knowledgeable about getting hold of Rachmaninoff’s first prelude (5)
I believe the answer is:
intro
'prelude' is the definition.
(I've seen this in another clue)
'knowledgeable about getting hold of rachmaninoff's first' is the wordplay.
'knowledgeable about' becomes 'into' (I've seen this before).
'getting hold of' indicates putting letters inside.
'rachmaninoff's first' becomes 'r' (1st letter of 'rachmaninoff').
'into' enclosing 'r' is 'INTRO'.
(Other definitions for intro that I've seen before include "Opening instrumental passage of song" , "overture" , "Prologue, opening (abbr.)" , "Song's opening" , "preface" .)