Bought at the grocer's, is the horse mad about them? (7)

'bought at the grocer's' is the definition.
I can't judge whether this defines the answer.

'the horse mad about them?' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'horse' becomes 'cob' (type of horse).
'mad' becomes 'nus' (I can't justify this - if you can you should believe this answer much more).
'about them?' indicates putting letters inside.
'cob'+'nus'='cobnus'
't' inserted inside 'cobnus' is 'COBNUTS'.

'is' is the link.

Can you help me to learn more?