Harbour we hear associated with a lock (3)
I believe the answer is:
key
'lock' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for key that I've seen before include "Essential; solution" , "Coral reef off southern coast of Florida" , "It can open" , "Lock opener; low island" , "Crucial arch-stone" .)