Crusty, edible cases a problem for mince pie (4)
I believe the answer is:
stye
'a problem for mince pie' is the definition.
(mince pie is Cockney rhyming slang for eye)
'crusty edible cases' is the wordplay.
'cases' indicates the answer is hidden within the clue.
'STYE' is hidden within 'crusty edible'.
(Other definitions for stye that I've seen before include "Eye problem" , "producing eyesore" , "Eyelid affliction" , "Ocular inflammation" , "Eyelid problem" .)