Agree to change after millions becomes inadequate (6)
I believe the answer is:
meagre
'inadequate' is the definition.
(thesaurus)
'agree to change after millions' is the wordplay.
'to change' indicates anagramming the letters.
'after' says to put letters next to each other.
'millions' becomes 'm' (abbreviation**).
'agree' anagrammed gives 'eagre'.
'eagre' after 'm' is 'MEAGRE'.
'becomes' is the link.
(Other definitions for meagre that I've seen before include "skinny" , "Small, insufficient" , "Inadequate, scanty" , "Thin on the ground, scarce" , "lacking in strength" .)