Instrument with which old boy regularly trimmed novel (4)
I believe the answer is:
oboe
'instrument' is the definition.
(type of musical instrument)
'old boy regularly trimmed novel' is the wordplay.
'old boy' becomes 'ob' (abbreviation).
'regularly trimmed' indicates alternate letters.
The alternate letters of 'novel' are 'oe'.
'ob'+'oe'='OBOE'
'with which' is the link.
(Other definitions for oboe that I've seen before include "Double-reeded woodwind instrument" , "Double-reed wind instrument" , "Woodwind instrument of treble pitch" , "Source of music" , "source of notes" .)