Instrument that’s long in a way (6)
I believe the answer is:
spinet
'instrument' is the definition.
(spinet is a kind of instrument)
'long in a way' is the wordplay.
'long' becomes 'pine' (synonyms).
'in' indicates putting letters inside.
'a way' becomes 'st'.
'pine' inserted into 'st' is 'SPINET'.
'that's' acts as a link.
(Other definitions for spinet that I've seen before include "Step in with old harpsichord" , "Old keyboard instrument" , "Step in with old musical instrument" , "Small 12" , "organ" .)