Instrument provided by chap in post office (5)
I believe the answer is:
piano
'instrument' is the definition.
(musical instrument)
'chap in post office' is the wordplay.
'chap' becomes 'ian' (man's name).
'in' is an insertion indicator.
'post office' becomes 'po' (abbreviation).
'ian' going within 'po' is 'PIANO'.
'provided by' is the link.
(Other definitions for piano that I've seen before include "Musical instrument - play quietly" , "Franz Liszt's instrument" , "Keyboard instrument" , "key holder" , "Opposite of forte" .)