Shark, unusually ferocious, gobbling up one’s nanny (9)
I believe the answer is:
nursemaid
'nanny' is the definition.
(synonyms)
'shark unusually ferocious gobbling up one's' is the wordplay.
'shark' becomes 'nurse' (nurse shark is a kind of shark).
'unusually ferocious' becomes 'mad'.
'gobbling up' indicates putting letters inside.
'one' becomes 'i' (roman numeral).
'mad' placed around 'i' is 'maid'.
'nurse'+'maid'='NURSEMAID'
(Other definitions for nursemaid that I've seen before include "She may attend to babe" , "One looking after children" , "Be overprotective towards (someone)" , "Woman tending a child or children" , "Woman employed to look after children" .)