Extreme custom Dorset regularly observed (6)

I believe the answer is:

utmost

'extreme' is the definition.
(I know that extreme can be written as utmost)

'custom dorset regularly observed' is the wordplay.
'regularly observed' indicates alternate letters (letters selected at regular intervals).
'custom'+'dorset'='customdorset'
The alternating letters of 'customdorset' are 'UTMOST'.

(Other definitions for utmost that I've seen before include "At furthest limit" , "Farthest" , "(To the) maximum extent" , "Furthest or maximum possible" , "Paramount" .)

I've seen this clue in The Telegraph.

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