You don't want her overtaking you! (7)
I believe the answer is:
nemesis
'overtaking you' is the definition.
The definition suggests a singular noun which matches the answer.
'you don't want her' is the wordplay.
I cannot quite understand how this works, but
'her' could be 'em' and 'em' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for nemesis that I've seen before include "Fate" , "Retribution personified" , "Figure of retribution" , "Something causing misery (or death) - Meissen (anag)" , "The Greek goddess of divine retribution and vengeance" .)