Man stumbled on old wife (6)
I believe the answer is:
fellow
'man' is the definition.
(I know that fellow is a type of man)
'stumbled on old wife' is the wordplay.
'stumbled' becomes 'fell' (I've seen this in another clue).
'on' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'wife' becomes 'w' (abbreviation).
'fell'+'o'+'w'='FELLOW'
(Other definitions for fellow that I've seen before include "Neglected child" , "Companion or associate" , "Academic chap?" , "Bloke; senior member of college" , "The other of a pair - just a chap" .)