Set about, therefore, getting an instrument (8)
I believe the answer is:
resolute
'set' is the definition.
(I've seen this before)
'about therefore getting an instrument' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'therefore' becomes 'so' ('so' can mean 'consequently' or 'therefore').
'getting' means one lot of letters go next to another.
'an instrument' becomes 'lute' (lute is a kind of instrument).
're'+'so'+'lute'='RESOLUTE'
(Other definitions for resolute that I've seen before include "Purposeful and unwavering" , "Reels out (anag.)" , "without flinching" , "Firm-minded" , "'Determined, dead set on (8)'" .)