Exercise poet doesn't finish - he wants everything perfect (6)
I believe the answer is:
pedant
'he wants everything perfect' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'exercise poet doesn't finish' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'poet' becomes 'dante' (13th- and 14th-century Italian poet).
'doesn't finish' means to remove the last letter.
'dante' with its last letter taken away is 'dant'.
'pe'+'dant'='PEDANT'
Can you help me to learn more?
(Other definitions for pedant that I've seen before include "Pentad (anag.)" , "Particular type" , "Know-all who insists on strict observance of rules" , "Purist" , "quibbler" .)