To begin with, had done wrong in having nagged at (7,2)
I believe the answer is:
started on
'to begin with' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'done wrong in having nagged at' is the wordplay.
I cannot quite see how this works, but
an anagram of 'done' is 'edon' which is present in the answer.
The remaining letters 'start' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'had' is the link.
Can you help me to learn more?
(Another definition for started on that I've seen is " Picked quarrel".)