The reason that keeps one writing fantasy (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')
'the reason that keeps one writing' is the wordplay.
'the reason' becomes 'why' (why something happened).
'that keeps' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'writing' becomes 'ms' (abbreviation for manuscript).
'i'+'ms'='ims'
'why' enclosing 'ims' is 'WHIMSY'.
(Other definitions for whimsy that I've seen before include "Playfully quaint behaviour" , "Something fanciful" , "Quaint - odd" , "Capricious notion" , "Capricious idea - sounds like Sayers' sleuth" .)