A woman of unique value (3)
I believe the answer is:
eva
'a woman' is the definition.
(woman's name)
'of unique value' is the wordplay.
'of' indicates a hidden word.
'EVA' is hidden in the letters of 'unique value'.
(Other definitions for eva that I've seen before include "Woman" , "Miss Peron, perhaps" , "-- Braun, Hitler mistress" , "- - , nee Braun" , "-- Cassidy, singer, 1963-96" .)