Terribly eager to concur (5)
I believe the answer is:
agree
'concur' is the definition.
(synonyms)
'terribly eager' is the wordplay.
'terribly' indicates an anagram.
'eager' is an anagram of 'AGREE'.
'to' is the link.
(Other definitions for agree that I've seen before include "Be of one mind" , "Concur, coincide" , "grant" , "Have the same idea" , "Enjoy view in common" .)