Waugh has heartless lady in flat (6)
I believe the answer is:
evelyn
'waugh' is the definition.
(novelist Evelyn Waugh)
'heartless lady in flat' is the wordplay.
'heartless' means to remove the middle letters.
'in' is an insertion indicator.
'flat' becomes 'even' (level).
'lady' with its middle removed is 'ly'.
'ly' placed into 'even' is 'EVELYN'.
'has' is the link.
(Other definitions for evelyn that I've seen before include "'Brideshead Revisited' - novel by . . . . . . Waugh" , "Boy's or girl's name" , "Authors John ---, --- Waugh" , "John --, gardener and diarist" , "London diarist" .)