It is a blemish on the most radiant of faces (7)
I believe the answer is:
sunspot
'it is a blemish' is the definition.
(I have seen 'Solar blemish ' mean 'sunspot' so perhaps 'blemish' could also mean 'sunspot')
'on the most radiant of faces' is the wordplay.
'on' means one lot of letters go next to another.
'the most' becomes 'tops' (I've seen this in another clue).
'radiant of' becomes 'nus' (I can't explain this - if you can you should believe this answer much more).
'faces' says the letters should be written in reverse (some letters turn towards or face others).
'tops' put next to 'nus' is 'topsnus'.
'topsnus' in reverse letter order is 'SUNSPOT'.
Can you help me to learn more?
(Other definitions for sunspot that I've seen before include "holiday resort" , "warm place" , "Cool darker patch on our star" , "Solar cool patch" , "Dark cool patch on the surface of our nearest star" .)