Fantasy question about single manuscript (6)
I believe the answer is:
whimsy
'fantasy' is the definition.
(I have seen 'Gentle fantasy' mean 'whimsy' so perhaps 'fantasy' could also mean 'whimsy')
'question about single manuscript' is the wordplay.
'question' becomes 'why' (why is a question word).
'about' is an insertion indicator.
'single' becomes 'I' (I as in one).
'manuscript' becomes 'ms' (abbreviation for manuscript).
'i'+'ms'='ims'
'why' placed around 'ims' is 'WHIMSY'.
(Other definitions for whimsy that I've seen before include "Playfully quaint behaviour" , "Quaint fanciful humour" , "Fanciful thoughts" , "Capricious humour" , "Fanciful ideas" .)