Agree to differ on millions being inadequate (6)
I believe the answer is:
meagre
'inadequate' is the definition.
(thesaurus)
'agree to differ on millions' is the wordplay.
'to differ' indicates an anagram.
'on' says to put letters next to each other.
'millions' becomes 'm' (abbreviation**).
'agree' anagrammed gives 'eagre'.
'eagre' after 'm' is 'MEAGRE'.
'being' is the link.
(Other definitions for meagre that I've seen before include "Scarce, not plentiful" , "miserly" , "Inadequate, scanty" , "Lacking in quantity/quality" , "Lean, thin" .)