Iron Duke consumes this regularly — it's foul (5)
I believe the answer is:
fetid
'foul' is the definition.
(I know that foul can be written as fetid)
'iron duke consumes this regularly' is the wordplay.
'iron' becomes 'fe' (chemical symbol for iron).
'duke' becomes 'd' (abbreviation).
'consumes' indicates putting letters inside.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'this' are 'ti'.
'fe'+'d'='fed'
'fed' placed around 'ti' is 'FETID'.
'it's' acts as a link.
(Other definitions for fetid that I've seen before include "Noxious" , "Smelly through decay" , "Nauseating" , "Foul-smelling or stinking" , "Stinking, like animal breath" .)