Terribly eager to concur (5)
I believe the answer is:
agree
'concur' is the definition.
(synonyms)
'terribly eager' is the wordplay.
'terribly' is an anagram indicator.
'eager' anagrammed gives 'AGREE'.
'to' acts as a 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" .)