TV presenter has one opponent (4)
I believe the answer is:
anti
'opponent' is the definition.
(I've seen this before)
'tv presenter has one' is the wordplay.
'tv presenter' becomes 'ant' (one half of Ant and Dec).
'has' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral**).
'ant'+'i'='ANTI'
(Other definitions for anti that I've seen before include "Opposed to followers of Hitler" , "Person opposed" , "One who is against (something)" , "'No' voter" , "Prefix meaniing against" .)