Set off in silver on the edge of one's seat (4)
I believe the answer is:
agog
'on the edge of one's seat' is the definition.
(I've seen this in another clue)
'set off in silver' is the wordplay.
'set off' becomes 'go' (I've seen this in other clues).
'in' is an insertion indicator.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'go' placed within 'ag' is 'AGOG'.
(Other definitions for agog that I've seen before include "Eagerly excited" , "Eager, avid" , "Mouth open" , "Excitedly eager" , "Expectant and wide-eyed" .)