Dumbest fool, finally moving, nearly fell (8)
I believe the answer is:
stumbled
'nearly fell' is the definition.
'dumbest fool finally moving' is the wordplay.
'finally' suggests the final letters.
'moving' indicates anagramming the letters.
The final letter of 'fool' is 'l'.
'dumbest'+'l'='dumbestl'
'dumbestl' is an anagram of 'STUMBLED'.
(Other definitions for stumbled that I've seen before include "Lost one's footing" , "Momentarily tripped" , "Missed a step and nearly fell" , "Missed a step and almost fell" , "Tripped, faltered" .)