Spot that woman’s instrument (6)
I believe the answer is:
zither
'instrument' is the definition.
(zither is a kind of instrument)
'spot that woman's' is the wordplay.
'spot' becomes 'zit' (spot on one's body).
'that woman' becomes 'her' ('her' refers to a woman).
'zit'+'her'='ZITHER'
(Other definitions for zither that I've seen before include "Instrument played on the knees" , "Metal-stringed soundbox" , "Stringed instrument placed on the knee" , "Harp-like instrument" , "music producer" .)