Just a single right can be a knockout! (4)
I believe the answer is:
oner
'a knockout' is the definition.
(I've seen this in another clue)
'just a single right' is the wordplay.
'just a single' becomes 'one' (only one).
'right' becomes 'r' (common abbreviation).
'one'+'r'='ONER'
'can be' is the link.
(Other definitions for oner that I've seen before include "One pound note; big lie" , "A PS1 note (colloq.)" , "Hit" , "Outstanding person" , "Heavy blow; 1 note" .)