Prioress is terrified about a nun (6)
I believe the answer is:
sister
'a nun' is the definition.
(a nun might be called sister)
'prioress is terrified about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'SISTER' can be found hidden inside 'prioress is terrified'.
(Other definitions for sister that I've seen before include "habits' - one who has them" , "A kinswoman" , "woman in hospital" , "Tender" , "twin?" .)