Try to gain access, which is an infringement of the rules (5-2)
I believe the answer is:
knock-on
'infringement of the rules' is the definition.
'knock on' can be an answer for 'infringement' (I have seen 'Infringement of rules' mean 'knock on' so perhaps 'infringement' could also mean 'knock on'). I am not sure about the 'of the rules' bit.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for knock-on that I've seen before include "Foul at rugby" , "Secondary effect" , "Fail to catch cleanly (rugby)" , "Consequent (effect)" , "Fumble (rugby pass)" .)