Violate popular part of Edinburgh Festival (8)
I believe the answer is:
infringe
'violate' is the definition.
(I know that infringe is a type of violate)
'popular part of edinburgh festival' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'part of edinburgh festival' becomes 'fringe' (I can't explain this - if you can you should give a lot more credence to this answer).
'in'+'fringe'='INFRINGE'
Can you help me to learn more?
(Other definitions for infringe that I've seen before include "Encroach on (a right)" , "Refining so as to act against rule or agreement" , "Break (rule)" , "Encroach, impinge" , "Cross boundary" .)