A letter in retinol is perfectly reversed (7)
I believe the answer is:
epsilon
'a letter' is the definition.
(epsilon is a kind of letter)
'in retinol is perfectly reversed' is the wordplay.
'in' indicates a hidden word.
'reversed' shows that the letters should be reversed in order.
'nolispe' can be found hidden inside 'retinol is perfectly'.
'nolispe' in reverse letter order is 'EPSILON'.
(Other definitions for epsilon that I've seen before include "small number mispronounced, almost completely the wrong way round" , "Foreign character" , "5th letter of the Greek alphabet" , "Greek short E" , "foreign letter" .)