Number includes a second error (6)
I believe the answer is:
groans
'number includes a second error' is the definition.
I can't judge whether this defines the answer.
'number includes a' is the wordplay.
'number' becomes 'gros' (I can't explain this - if you can you should give a lot more credence to this answer).
'includes' is an insertion indicator.
'a' becomes 'an'.
'gros' going around 'an' is 'GROANS'.
Can you help me to learn more?
(Other definitions for groans that I've seen before include "Sounds of pain or disapproval" , "Grumbles" , "Complains" .)