This writer's for and against spontaneous routine (6)
I believe the answer is:
improv
'spontaneous routine' is the definition.
(improvised performance)
'this writer's for and against' is the wordplay.
'this writer's' becomes 'i'm' (referring to the person writing the clues).
'for' becomes 'pro' (as in being 'pro' something).
'and' means one lot of letters go next to another.
'against' becomes 'v' (versus).
'im'+'pro'+'v'='IMPROV'
(Other definitions for improv that I've seen before include "Off-the-cuff comedy" , "Stand-up comedy, perhaps" , "off-the-cuff humour" , "little ad-libbing" .)