Sweetheart not heartless with cool allure (6)
I believe the answer is:
entice
'allure' is the definition.
(thesaurus)
'sweetheart not heartless with cool' is the wordplay.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'heartless' suggests removing the centre.
'with' says to put letters next to each other.
'cool' becomes 'ice' (icing is a kind of cooling).
'not' with its middle taken out is 'nt'.
'e'+'nt'+'ice'='ENTICE'
(Other definitions for entice that I've seen before include "alluring" , "Tempt, lead astray" , "Attract with offers" , "to offer temptation" , "Attract with inducement" .)