An infringement of what marks the boundary (7)
I believe the answer is:
offence
'an infringement' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for offence that I've seen before include "Displeasure" , "Violation" , "Hurt; military attack" , "Misdeed" , "Criminal action" .)