A woman who's unusually sure -- no act! (9)
I believe the answer is:
courtesan
'a woman' is the definition.
(courtesan is a kind of woman)
'unusually sure no act' is the wordplay.
'unusually' is an anagram indicator.
'sure'+'no'+'act'='surenoact'
'surenoact' with letters rearranged gives 'COURTESAN'.
'who's' is the link.
(Other definitions for courtesan that I've seen before include "loose woman" , "paramour" , "Kept woman" , "High-class prostitute" , "odalisque" .)