It's mouldy according to gentle sheep and dunderhead (8)

I believe the answer is:

mildewed

'it's mouldy' is the definition.
(thesaurus)

'gentle sheep and dunderhead' is the wordplay.
'gentle' becomes 'mild' (similar in meaning).
'sheep' becomes 'ewe' (a ewe is a female sheep).
'and' says to put letters next to each other.
'dunderhead' becomes 'd' (head letter of 'dunder').
'mild'+'ewe'+'d'='MILDEWED'

'according to' is the link.

(Another definition for mildewed that I've seen is " Coated with white fungus due to damp".)

I've seen this clue in The Irish News.

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