Almost stumbled on crook (5)
I believe the answer is:
felon
'crook' is the definition.
(I know that crook can be written as felon)
'almost stumbled on' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'stumbled' becomes 'fell' (to stumble is to fall over).
'fell' with its final letter taken away is 'fel'.
'fel'+'on'='FELON'
(Other definitions for felon that I've seen before include "Person who has committed a serious crime" , "Mighty" , "Has conviction" , "Person guilty of grave crime" , "convict" .)