On the edge of one's seat as grumpy old gentleman starts (4)
I believe the answer is:
agog
'on the edge of one's seat' is the definition.
(I've seen this in another clue)
'as grumpy old gentleman starts' is the wordplay.
'starts' suggests taking the first letters.
The first letters of 'as grumpy old gentleman' is 'AGOG'.
(Other definitions for agog that I've seen before include "Eager and wide-eyed" , "Unable to wait" , "State of wide-eyed wonder" , "Expectant, eager" , "on the edge of your seat" .)