Agreement is sealed by Italian, one wanting perfection (8)
I believe the answer is:
idealist
'one wanting perfection' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'agreement is sealed by italian' is the wordplay.
'agreement' becomes 'deal' (deal is a kind of agreement).
'sealed by' is an insertion indicator.
'italian' becomes 'it' (abbreviation for Italian).
'deal'+'is'='dealis'
'dealis' placed inside 'it' is 'IDEALIST'.
Can you help me to learn more?
(Other definitions for idealist that I've seen before include "One pursuing perfection" , "philosopher" , "One sees things as they should be rather than how they are" , "Utopian" , "Person with high and unselfish standards" .)