Amazed to have stumbled (9)
I believe the answer is:
staggered
'amazed' is the definition.
(staggering is a kind of amazing)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for staggered that I've seen before include "Amazed" , "Deeply shocked or walked unsteadily as if about to fall" , "Moved unsteadily, being deeply shocked" , "Arranged to take place at intervals" , "Astonished" .)