Old woman on run causes harm (3)
I believe the answer is:
mar
'harm' is the definition.
(I've seen this before)
'old woman on run' is the wordplay.
'old woman' becomes 'ma' (both can mean someone's mother).
'on' is a charade indicator (letters next to each other).
'run' becomes 'r' (cricket abbreviation).
'ma'+'r'='MAR'
'causes' acts as a link.
(Other definitions for mar that I've seen before include "'Damage, impair (3)'" , "Ram into and spoil" , "A windy month" , "Spoil stuff" , "hurt" .)