Ready to pop back in to get the instrument (6)
I believe the answer is:
spinet
'the instrument' is the definition.
(spinet is a historical type of musical instrument)
'ready to pop back in' is the wordplay.
'ready to' becomes 'set' (synonyms).
'pop' becomes 'nip' ('nip out' and 'pop out' can both mean to go out).
'back' says the letters should be written backwards.
'in' indicates putting letters inside.
'nip' reversed gives 'pin'.
'set' placed around 'pin' is 'SPINET'.
'to get' is the link.
(Other definitions for spinet that I've seen before include "Step in with old musical instrument" , "Step in with old harpsichord" , "A small harpsichord of old" , "Small 12" , "Instep (anag)" .)