Scotsman in Post Office to get instrument (5)
I believe the answer is:
piano
'instrument' is the definition.
(piano is a kind of instrument)
'scotsman in post office' is the wordplay.
'scotsman' becomes 'ian' (common Scottish name).
'in' means one lot of letters goes inside another.
'post office' becomes 'po'.
'ian' put inside 'po' is 'PIANO'.
'to get' is the link.
(Other definitions for piano that I've seen before include "quiet passage" , "In musical notation, this means softly" , "Musical instrument - softly, please" , "Direction in music (1): relatively softly" , "Softly, musically speaking" .)