Long way out musical instrument (6)
I believe the answer is:
spinet
'musical instrument' is the definition.
(spinet is a kind of musical instrument)
'long way out' is the wordplay.
'long' becomes 'pine' ('pine' can be a synonym of 'long'**).
'way' becomes 'st'.
'out' indicates putting letters inside.
'pine' placed into 'st' is 'SPINET'.
(Other definitions for spinet that I've seen before include "Instep (anag)" , "Type of early harpsichord" , "Old keyboard instrument" , "organ" , "Instrument resembling a small harpsichord" .)