Woman to ring back to get some varnish (7)
I believe the answer is:
shellac
'some varnish' is the definition.
(shellac varnish is a kind of varnish)
'woman to ring back' is the wordplay.
'woman' becomes 'she' ().
'to ring' becomes 'call' (call on the phone).
'back' is a reversal indicator.
'call' reversed gives 'llac'.
'she'+'llac'='SHELLAC'
'to get' is the link.
(Other definitions for shellac that I've seen before include "Varnish ingredient" , "Resin used for varnish, formerly for records" , "Defeat" , "Resin from trees" , "Varnish derived from insect secretion" .)