Catch and knock out a redhead! (4)
I believe the answer is:
trap
'catch' is the definition.
(trapping is a kind of catching)
'knock out a redhead' is the wordplay.
'knock' becomes 'tap' ('tap' can be a synonym of 'knock').
'out' is an insertion indicator.
'a redhead' becomes 'r' (the head/first letter of 'red').
'tap' enclosing 'r' is 'TRAP'.
'and' acts as a link.
(Other definitions for trap that I've seen before include "Device to catch prey" , "Means of transport - sometimes sprung!" , "Control ball" , "Confine" , "Catch; two-wheeled carriage" .)