Hire purchase securing a rookie’s first musical instrument (4)
I believe the answer is:
harp
'musical instrument' is the definition.
(I know that harp is a type of free-reed instrument)
'hire purchase securing a rookie's first' is the wordplay.
'hire purchase' becomes 'HP' (abbreviation).
'securing' means one lot of letters goes inside another.
'rookie's first' becomes 'r' (1st letter of 'rookie').
'a'+'r'='ar'
'hp' going around 'ar' is 'HARP'.
(Other definitions for harp that I've seen before include "Heavenly instrument?" , "David's instrument" , "semitone higher, abandoning intro" , "Musical symbol" , "Chordophone" .)