Legend in dock according to hearsay (3)
I believe the answer is:
key
'legend' is the definition.
(legend can mean a key on a map or diagram)
'dock according to hearsay' is the wordplay.
'dock' becomes 'quay' (quay is a kind of dock).
'according to hearsay' shows a homophone (sound like).
'quay' is a homophone of 'KEY'.
'in' is the link.
(Other definitions for key that I've seen before include "Lock-opener" , "It can open" , "Coral reef off southern coast of Florida" , "Lock undoer" , "Crucial; island" .)