Ring that used to be a lady's (6)
I believe the answer is:
washer
'ring' is the definition.
(I've seen this before)
'used to be a lady's' is the wordplay.
'used to be' becomes 'was' (ie used to be or was in the past).
'a lady' becomes 'her' (her refers to a woman).
'was'+'her'='WASHER'
'that' acts as a link.
(Other definitions for washer that I've seen before include "Ring under a nut" , "Seal preventing leakage" , "Plumbing seal" , "Metal disc for hawser" , "Seal to prevent leakage" .)