But Kenneth was no extra (4)
I believe the answer is:
more
'extra' is the definition.
(both can mean additional)
'but kenneth was no' is the wordplay.
I cannot really see how this works, but
'no' could be 'o' (resembles 0 - 'no' can mean 'zero') and 'o' is found within the answer.
The remaining letters 'mre' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for more that I've seen before include "To a greater extent" , "author" , "A greater amount" , "victim of Henry" , "Writer" .)