First pair often build a barrier around it, in violation of the law (7)
I believe the answer is:
offence
'violation of the law' is the definition.
'offence' can be an answer for 'violation' (synonyms). I'm not certain of the 'of the law' bit.
'first pair often build a barrier around it' is the wordplay.
I cannot really see how this works, but
'barrier' could be 'fence' (fence is a kind of barrier) and 'fence' is found within the answer.
'around' could be 'o' (a round letter) and 'o' is found in the answer.
A single letter 'f' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for offence that I've seen before include "Misdeed" , "Crime - umbrage" , "The handler" , "Cause of annoyance - crime" , "Resentment, hurt" .)