Gate of harbour by a lake (6)
I believe the answer is:
portal
'gate' is the definition.
(thesaurus)
'harbour by a lake' is the wordplay.
'harbour' becomes 'port' (I've seen this before).
'by' says to put letters next to each other.
'lake' becomes 'l'.
'port'+'a'+'l'='PORTAL'
'of' is the link.
(Other definitions for portal that I've seen before include "An imposing gateway" , "Grand and imposing entrance" , "Special website bringing together information from diverse sources in a uniform way" , "An imposing door or gate" , "An imposing gate or doorway" .)