Attractiveness of heartless girl having secret affair (7)
I believe the answer is:
glamour
'attractiveness' is the definition.
(I've seen this before)
'heartless girl having secret affair' is the wordplay.
'heartless' suggests removing the centre.
'having' is a charade indicator (letters next to each other).
'secret affair' becomes 'amour' (amour can mean a lover).
'girl' with its middle taken out is 'gl'.
'gl'+'amour'='GLAMOUR'
'of' is the link.
(Other definitions for glamour that I've seen before include "Charm and allure" , "Fascinating beauty" , "Voluptuous allure" , "Beauty created by clothes and make-up" , "Attractive and exciting quality" .)