Performance needs superior accomplice, in fact (8)
I believe the answer is:
actually
'in fact' is the definition.
('actually' can be a synonym of 'in fact')
'performance needs superior accomplice' is the wordplay.
'performance' becomes 'act' (act is a kind of performance**).
'needs' says to put letters next to each other.
'superior' becomes 'u' (I've seen this in other clues).
'accomplice' becomes 'ally' (I've seen this before).
'act'+'u'+'ally'='ACTUALLY'
(Other definitions for actually that I've seen before include "In reality" , "Really, in fact" , "A lay cult may be existing in fact" , "As a matter of fact" , "Really existing" .)