In record time, one performing duet shows incompetence (10)
I believe the answer is:
ineptitude
'incompetence' is the definition.
(I've seen this before)
'in record time one performing duet' is the wordplay.
'record' becomes 'ep'.
'time' becomes 't'.
'one' becomes 'i' (Roman numeral).
'performing' indicates an anagram.
'duet' is an anagram of 'tude'.
'in'+'ep'+'t'+'i'+'tude'='INEPTITUDE'
'shows' acts as a link.
(Other definitions for ineptitude that I've seen before include "Incompetence" , "Awkwardness, clumsiness" , "lack of competence" .)