Unusual case for experts (4)
I believe the answer is:
aces
'for experts' is the definition.
(ace is a kind of expert)
'unusual case' is the wordplay.
'unusual' indicates anagramming the letters.
'case' anagrammed gives 'ACES'.
(Other definitions for aces that I've seen before include "holes in one" , "Flying war heroes" , "wartime pilots?" , "excellent service" , "Experts (in their field)" .)