He's all doltish, confused: that's long-established (3,2,3,5)

I believe the answer is:

old as the hills

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'long-established' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'he's all doltish confused' is the wordplay.
'confused' is an anagram indicator.
'hes'+'all'+'doltish'='hesalldoltish'
'hesalldoltish' is an anagram of 'OLD AS THE HILLS'.

'that's' acts as a link.

Can you help me to learn more?

(Other definitions for old as the hills that I've seen before include "antique" , "Ancient" .)

I've seen this clue in The Everyman puzzle in The Observer.

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