Unprepared stuff the writer's both for and against (6)
I believe the answer is:
improv
'unprepared stuff the' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'writer's both for and against' is the wordplay.
'writer's' becomes 'im' ('writer's' could mean 'the crossword setter is').
'both for' becomes 'pro' (as in being 'pro' something. I am not sure about the 'both' bit.).
'and' says to put letters next to each other.
'against' becomes 'v' (abbreviation for 'versus').
'im'+'pro'+'v'='IMPROV'
Can you help me to learn more?
(Other definitions for improv that I've seen before include "Stand-up comedy, perhaps" , "Off-the-cuff comedy" , "little ad-libbing" , "off-the-cuff humour" .)