Promise a superior form of rug (5)

I believe the answer is:

augur

'promise' is the definition.
(auguring is a kind of promising)

'a superior form of rug' is the wordplay.
'superior' becomes 'u' (I've seen this in other clues).
'form of' is an anagram indicator (the letters need to be in another 'form').
'rug' is an anagram of 'gur'.
'a'+'u'+'gur'='AUGUR'

(Other definitions for augur that I've seen before include "Presage, foretell" , "Roman diviner" , "Portend; Roman official" , "Prophesy" , "A guru (anag)" .)

I've seen this clue in the Evening Standard.

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