Troublemaker is not up to scratch on English horse (3,3)
I believe the answer is:
bad egg
'troublemaker is not up to scratch' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'english horse' is the wordplay.
I cannot quite see how this works, but
'english' could be 'e' (abbreviation) and 'e' is found within the answer.
'horse' could be 'gg' and 'gg' is present in the answer.
The remaining letters 'bad' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'on' acts as a link.
Can you help me to learn more?
(Other definitions for bad egg that I've seen before include "Untrustworthy type (smelling rotten?)" , "A rotten ovoid" , "Nasty fellow like addled ovum" , "Dishonest person like addled ovoid" , "Worthless person" .)