Genuine source of arbitration disputed in the TUC (9)
I believe the answer is:
authentic
'genuine' is the definition.
('authentic' can be similar in meaning to 'genuine')
'source of arbitration disputed in the tuc' is the wordplay.
'source of' suggests taking the first letters.
'disputed' indicates an anagram.
The first letter of 'arbitration' is 'a'.
'in'+'the'+'tuc'='inthetuc'
'inthetuc' with letters rearranged gives 'uthentic'.
'a'+'uthentic'='AUTHENTIC'
(Other definitions for authentic that I've seen before include "Ian, the cut is genuine" , "reliable" , "trustworthy" , "Bona fide" , "24 [LEGIT]" .)