Instrument fixed with nail inside (6)
I believe the answer is:
spinet
'instrument' is the definition.
(spinet is a historical type of musical instrument)
'fixed with nail inside' is the wordplay.
'fixed with' becomes 'set' (both can mean decided or determined. I am not sure about the 'with' bit.).
'nail' becomes 'pin' (both can mean a sharp object).
'inside' means one lot of letters goes inside another.
'set' placed around 'pin' is 'SPINET'.
Can you help me to learn more?
(Other definitions for spinet that I've seen before include "Instrument resembling a small harpsichord" , "Step in with old harpsichord" , "Early keyboard instrument" , "Harpsichord-like keyboard instrument" , "Step in with old musical instrument" .)