A nasty blemish is found in trademark - one's not looking one's best with it (11)
I believe the answer is:
astigmatism
'one's not looking one's best with it' is the definition.
I can't tell whether this defines the answer.
'a nasty blemish is found in trademark' is the wordplay.
'nasty blemish' becomes 'stigma' (stigma is a kind of blemish. I am not sure about the 'nasty' bit.).
'found in' means one lot of letters goes inside another.
'trademark' becomes 'tm' (abbreviation for trademark).
'is' placed inside 'tm' is 'tism'.
'a'+'stigma'+'tism'='ASTIGMATISM'
Can you help me to learn more?
(Other definitions for astigmatism that I've seen before include "Wonkiness of vision" , "Sight distortion" , "Eye lens defect" , "Distortion of vision" .)