Crossword Genius

Say Tom's order is greatest (9)

Ross

I believe the answer is:

uttermost

'greatest' is the definition.
(thesaurus)

'say tom's order' is the wordplay.
'say' becomes 'utter' (I've seen this before).
'order' indicates anagramming the letters.
'toms' anagrammed gives 'most'.
'utter'+'most'='UTTERMOST'

'is' is the link.

(Other definitions for uttermost that I've seen before include "best" , "Maximum - extremity" , "Extreme" , "at the furthest limit" , "Last" .)

I've seen this clue in The Mirror.
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