The speaker's for and against ad-libbing (6)
I believe the answer is:
improv
'ad-libbing' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'the speaker's for and against' is the wordplay.
'the speaker's' becomes 'im' (I can't justify this - if you can you should believe this answer much more).
'for' becomes 'pro' (I've seen this before).
'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" , "little ad-libbing" , "Off-the-cuff comedy" , "off-the-cuff humour" .)