Crack, as a case (5)
I believe the answer is:
solve
'crack as a case' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for solve that I've seen before include "Deal with (a problem)" , "Settle" , "Get answer to (a clue)" , "Work out answer to puzzle" , "Work out (a clue)" .)