A little retinol is purifying retired Greek character (7)
I believe the answer is:
upsilon
'greek character' is the definition.
(letter from the Greek alphabet)
'a little retinol is purifying retired' is the wordplay.
'a little' says the answer is hidden in the clue (a little bit of the clue).
'retired' says the letters should be written backwards.
'nolispu' is hidden within 'retinol is purifying'.
'nolispu' written backwards gives 'UPSILON'.
(Other definitions for upsilon that I've seen before include "Greek character" , "Greek 'u'" , "Character from abroad" , "Greek letter" .)