Total zero at sport (4)
I believe the answer is:
sumo
'sport' is the definition.
(I know that sumo is a type of sport)
'total zero' is the wordplay.
'total' becomes 'sum' (synonyms).
'zero' becomes 'O' (O resembles 0).
'sum'+'o'='SUMO'
'at' acts as a link.
(Other definitions for sumo that I've seen before include "Style of traditional Japanese wrestling" , "Japanese wrestling sport of over-weight men" , "Japanese form of wrestling" , "foreign form of fighting" , "Grappling" .)