Add nothing for form of wrestling (4)
I believe the answer is:
sumo
'form of wrestling' is the definition.
(I've seen this before)
'add nothing' is the wordplay.
'add' becomes 'sum' ('sum' can be a synonym of 'add').
'nothing' becomes 'o' (looks like zero - 0).
'sum'+'o'='SUMO'
'for' is the link.
(Other definitions for sumo that I've seen before include "Type of Japanese wrestling" , "Japanese wrestling sport of over-weight men" , "Style of traditional Japanese wrestling" , "foreign form of fighting" , "Jap. wrestling" .)