Second complainant to run off (7)
I believe the answer is:
scarper
'second complainant to run off' is the definition.
I can't tell whether this defines the answer.
'second complainant to run' is the wordplay.
'second' becomes 's' (common abbreviation).
'complainant' becomes 'carpe' (I can't explain this - if you can you should give a lot more credence to this answer).
'to run' becomes 'r' (cricket abbreviation).
's'+'carpe'+'r'='SCARPER'
Can you help me to learn more?
(Other definitions for scarper that I've seen before include "Clear off!" , "bolt" , "Escape" , "Damage" , "Take to one's heels, cut and run" .)