Break extraordinarily fine case for jewellery (8)
I believe the answer is:
infringe
'break' is the definition.
(I know this)
'extraordinarily fine case for jewellery' is the wordplay.
'extraordinarily' indicates anagramming the letters (I've seen this in other clues).
'case for' is an insertion indicator.
'jewellery' becomes 'ring' (ring is a kind of jewellery).
'fine' with letters rearranged gives 'infe'.
'infe' placed around 'ring' is 'INFRINGE'.
(Other definitions for infringe that I've seen before include "Encroach, impinge" , "Refining so as to act against rule or agreement" , "Neglect to obey" , "Break the rules" , "Violate" .)