A number copy a restriction (10)
I believe the answer is:
limitation
'a restriction' is the definition.
(I know that restriction can be written as limitation)
'a number copy' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'number' could be 'limit' (I have seen 'maximum number' mean 'limit' so perhaps 'number' could also mean 'limit') and 'limit' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for limitation that I've seen before include "lack of ability" , "Shortcoming" , "Rule restricting extent" , "Check" , "lack of talent" .)