The area designed for a mistress (7)
I believe the answer is:
hetaera
'mistress' is the definition.
(term for a mistress or courtesan)
'the area designed' is the wordplay.
'designed' indicates an anagram (designed with the letters).
'the'+'area'='thearea'
'thearea' is an anagram of 'HETAERA'.
'for a' acts as a link.
(Other definitions for hetaera that I've seen before include "paramour" , "Old pro" , "Courtesan" .)