Eager to go up after a key (4)
I believe the answer is:
agog
'eager' is the definition.
(I've seen this in another clue)
'go up after a key' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'after' is a charade indicator (letters next to each other).
'key' becomes 'g' (musical key).
'go' backwards is 'og'.
'a'+'g'='ag'
'og' after 'ag' is 'AGOG'.
'to' acts as a link.
(Other definitions for agog that I've seen before include "Eagerly excited" , "Eagerly awaiting" , "In a highly excited state" , "Unable to wait" , "On tenterhooks" .)