Fancy dress mostly ready-made (8)
I believe the answer is:
daydream
'fancy' is the definition.
(thesaurus)
'dress mostly ready-made' is the wordplay.
'dress' indicates anagramming the letters.
'mostly' means to remove the last letter.
'ready'+'made'='readymade'
'readymade' with its last letter taken away is 'readymad'.
'readymad' is an anagram of 'DAYDREAM'.
(Other definitions for daydream that I've seen before include "Diurnal reverie" , "A pleasant diurnal fantasy" , "Wool-gather" , "Diurnal reveries" , "Distracting thoughts" .)