Highly excited in the past, disappointing ultimately (4)
I believe the answer is:
agog
'highly excited' is the definition.
(I've seen this before)
'past disappointing ultimately' is the wordplay.
'past' becomes 'ago' ('ago' can be similar in meaning to 'past').
'ultimately' says to take the final letters.
The last letter of 'disappointing' is 'g'.
'ago'+'g'='AGOG'
'in the' acts as a link.
(Other definitions for agog that I've seen before include "Expecting" , "Excitedly interested" , "Full of excitement" , "Wide-eyed and surprised" , "on the edge of your seat" .)