Singular redhead, maybe a knockout! (4)
I believe the answer is:
oner
'a knockout' is the definition.
(I've seen this in another clue)
'singular redhead' is the wordplay.
'singular' becomes 'one' (synonyms).
'redhead' becomes 'r' (the head/first letter of 'red').
'one'+'r'='ONER'
'maybe' acts as a link.
(Other definitions for oner that I've seen before include "A PS1 note (colloq.)" , "expert" , "Unique person" , "A pound coin" , "Big lie; unique thing" .)