Is critical of strikes (6)
I believe the answer is:
knocks
'strikes' is the definition.
(knocking is a kind of striking)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for knocks that I've seen before include "You get bad experiences in the school of hard ..." , "Raps; criticises" , "Raps on the door" , "Reversals" , "Gentle taps" .)