Nightmare about dead Mary? On the contrary (8)
I believe the answer is:
daydream
'nightmare' is the definition.
(thesaurus)
'dead mary? on the contrary' is the wordplay.
'on' means one lot of letters go next to another.
'the contrary' indicates an anagram.
'dead'+'mary'='deadmary'
'deadmary' is an anagram of 'DAYDREAM'.
'about' acts as a link.
(Other definitions for daydream that I've seen before include "Distracting thoughts" , "(Indulge in) idle reverie" , "Indulge in a fantasy" , "Diurnal reverie" , "Diurnal reveries" .)