Laundress used to be a woman (6)
I believe the answer is:
washer
'laundress' is the definition.
(I've seen this before)
'used to be a woman' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'a woman' becomes 'her' (her refers to a woman).
'was'+'her'='WASHER'
(Other definitions for washer that I've seen before include "Joint-sealing ring" , "Leak-stopping ring" , "Metal ring - cleans clothes" , "Flat ring as seal" , "Cleaner" .)