Fiona evidently conceals birthmarks (5)
I believe the answer is:
naevi
'birthmarks' is the definition.
(I've seen this before)
'fiona evidently conceals' is the wordplay.
'conceals' indicates the answer is hidden within the clue.
'NAEVI' can be found hidden inside 'fiona evidently'.
(Other definitions for naevi that I've seen before include "Moles" , "Patches of red" , "Blemishes" , "Spots" , "Birthmarks" .)