Say Tom's order is greatest (9)
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" .)