Old boy regularly loses instrument (4)
I believe the answer is:
oboe
'instrument' is the definition.
(type of musical instrument)
'old boy regularly loses' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'loses' are 'oe'.
'ob'+'oe'='OBOE'
(Other definitions for oboe that I've seen before include "Slender double-reed instrument" , "source of notes" , "Woodwind instrument of treble pitch" , "music producer" , "it's blown" .)