Way to secure deal for instrument (6)
I believe the answer is:
spinet
'for instrument' is the definition.
(spinet is a historical type of musical instrument)
'way to secure deal' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'to secure' indicates putting letters inside (inserted letters are secured inside).
'deal' becomes 'pine' (deal can mean wood for building with eg pine).
'st' enclosing 'pine' is 'SPINET'.
(Other definitions for spinet that I've seen before include "Instrument resembling a small harpsichord" , "Instep (anag)" , "A small harpsichord of old" , "organ" , "Early keyboard instrument" .)