Stinking, due to a fatheaded change of diet (5)
I believe the answer is:
fetid
'stinking' is the definition.
(synonyms)
'a fatheaded change of diet' is the wordplay.
'a fatheaded' becomes 'f' (this could be a standard abbreviation I've not previously seen).
'change of' indicates anagramming the letters.
'diet' is an anagram of 'etid'.
'f'+'etid'='FETID'
'due to' is the link.
Can you help me to learn more?
(Other definitions for fetid that I've seen before include "Evil-smelling, stinking" , "Noxious" , "Smelly through decay" , "Reeking" , "Having a stale and unpleasant smell" .)