Experts at cracking a case? (4)
I believe the answer is:
aces
'experts' is the definition.
(ace is a kind of expert)
'cracking a case?' is the wordplay.
'cracking' is an anagram indicator.
'case' is an anagram of 'ACES'.
'at' is the link.
(Other definitions for aces that I've seen before include "never come back" , "Case of top cards" , "excellent service" , "Flying war heroes" , "Expert fliers" .)