Teacher eating starter of tasty porridge (4)
I believe the answer is:
stir
'porridge' is the definition.
(both are slang terms for prison)
'teacher eating starter of tasty' is the wordplay.
'teacher' becomes 'sir' (way of addressing a male teacher).
'eating' indicates putting letters inside.
'starter of' suggests taking the first letters.
The initial letter of 'tasty' is 't'.
'sir' enclosing 't' is 'STIR'.
(Other definitions for stir that I've seen before include "Arouse; mix" , "Whisk or mix" , "Rouse; prison (colloq.)" , "Whisk, blend" , "Create revolutionary movement" .)