Stumbled over a slang word for man (5)
I believe the answer is:
fella
'slang word for man' is the definition.
(non-standard spelling of 'fellow')
'stumbled over a' is the wordplay.
'stumbled' becomes 'fell' (to stumble is to fall over).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'fell'+'a'='FELLA'
(Other definitions for fella that I've seen before include "Guy" , "Bloke" , "Chap" .)