Nurse, among sharks, is terrifying (6)
I believe the answer is:
sister
'nurse' is the definition.
(a female nurse might be called sister)
'among sharks is terrifying' is the wordplay.
'among' indicates a hidden word.
'SISTER' is hidden within 'sharks is terrifying'.
(Other definitions for sister that I've seen before include "twin?" , "Relative in hospital?" , "Nurse, nun or relation" , "habits' - one who has them" , "Resist (anag) - nurse" .)