Shark in harbour (5)
I believe the answer is:
nurse
This clue is a double definition.
'shark' is the first definition.
(nurse shark is a kind of shark)
'harbour' is the second definition.
(I know that harbour can be written as nurse)
'in' is the link.
(Other definitions for nurse that I've seen before include "Nanny" , "Look after: entertain" , "Carry on - - (film)" , "Nourish" , "Professional carer" .)