Dobbin's diet (4)
I believe the answer is:
oats
'dobbin's diet' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oats that I've seen before include "Cereal for porridge" , "Sow wild ones in disreputable youth" , "Edible grass seeds" , "Porridge mix" , "Grains or porridge" .)