Crook case of beer delivered earlier (5)
I believe the answer is:
brill
'crook' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'case of beer delivered earlier' is the wordplay.
'case of' suggests removing the centre (outsides of).
'delivered earlier' becomes 'ill' (I am not sure about this - if you are sure you should believe this answer much more).
'beer' with its centre taken out is 'br'.
'br'+'ill'='BRILL'
Can you help me to learn more?
(Other definitions for brill that I've seen before include "marvellous" , "Fab -- fish" , "Flatfish; excellent" , "Great" , "Champion" .)