Spot the woman’s instrument (6)
I believe the answer is:
zither
'instrument' is the definition.
(zither is a kind of instrument)
'spot the woman's' is the wordplay.
'spot' becomes 'zit'.
'the woman' becomes 'her'.
'zit'+'her'='ZITHER'
(Other definitions for zither that I've seen before include "Stringed box, played with plectrum" , "music player" , "Instrument played on the knee" , "Harp-like instrument" , "Stringed instrument placed on the knee" .)