Stupid hate somehow dwindled outside (9)

I believe the answer is:

fatheaded

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'stupid' is the definition.
(similar in meaning)

'hate somehow dwindled outside' is the wordplay.
'somehow' indicates anagramming the letters.
'dwindled' becomes 'faded' (I can't justify this - if you can you should believe this answer much more).
'outside' is an insertion indicator (some letters go outside others).
'hate' anagrammed gives 'thea'.
'thea' put inside 'faded' is 'FATHEADED'.

Can you help me to learn more?

(Other definitions for fatheaded that I've seen before include "weakened" , "Stupid" , "Informally foolish with obese cranium" .)

I've seen this clue in The Daily Express.

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