Instrument of Scot in post office (5)
I believe the answer is:
piano
'instrument' is the definition.
(I know that piano is a type of stringed instrument)
'scot in post office' is the wordplay.
'scot' becomes 'ian'.
'in' indicates putting letters inside.
'post office' becomes 'po' (abbreviation).
'ian' put inside 'po' is 'PIANO'.
'of' acts as a link.
(Other definitions for piano that I've seen before include "Softly (mus.)" , "In musical notation, this means softly" , "Upright type?" , "entertain with this softly" , "Musical instrument - softly, please" .)