He wants a good job and no mistake (13)
I believe the answer is:
perfectionist
'he' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a good job and no mistake' is the wordplay.
I cannot really see how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is found in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found in the answer.
'no' could be 'o' (resembles 0 - 'no' can mean 'zero') and 'o' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'wants' is the link.
Can you help me to learn more?
(Other definitions for perfectionist that I've seen before include "One aiming for the best" , "One seeking the ideal" , "The Stickler" , "Striver for the ideal" , "One striving for an ideal standard" .)