The second of two races in genuine reversal (6)
I believe the answer is:
latter
'the second of two' is the definition.
(I've seen this before)
'races in genuine reversal' is the wordplay.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'in' is an insertion indicator.
'genuine' becomes 'real' ('real' is associated in meaning with 'genuine').
'reversal' shows that the letters should be reversed in order.
'tt' put inside 'real' is 'rettal'.
'rettal' written backwards gives 'LATTER'.
(Other definitions for latter that I've seen before include "Second-mentioned; recent" , "second one" , "Second mentioned of two" , "Last of those mentioned" , "Concluding" .)