Porter's regulation salad (8)
I believe the answer is:
coleslaw
'salad' is the definition.
(coleslaw is a kind of salad)
'porter's regulation' is the wordplay.
'porter's' becomes 'cole's' (musician Cole Porter).
'regulation' becomes 'law' (I've seen this in another clue).
'coles'+'law'='COLESLAW'
(Other definitions for coleslaw that I've seen before include "Salad of shredded vegetables" , "Shredded vegetable salad" , "plant-based food" , "Salad dish of dressed shredded cabbage" , "healthy food" .)