Experts break case (4)
I believe the answer is:
aces
'experts' is the definition.
(ace is a kind of expert)
'break case' is the wordplay.
'break' indicates an anagram.
'case' with letters rearranged gives 'ACES'.
(Other definitions for aces that I've seen before include "Ones or elevens in cards" , "Playing cards with single pips on their faces" , "Flying war heroes" , "Experts (in their field)" , "Two will make deuce" .)