Better finish off performance that's not been prepared (6)
I believe the answer is:
improv
'performance that's not been prepared' is the definition.
(improvised performance)
'better finish off' is the wordplay.
'better' becomes 'improve' (synonyms).
'finish off' means to remove the last letter.
'improve' with its last letter taken off is 'IMPROV'.
(Other definitions for improv that I've seen before include "Off-the-cuff comedy" , "off-the-cuff humour" , "little ad-libbing" , "Stand-up comedy, perhaps" .)