Dockland area with a large entrance (6)
I believe the answer is:
portal
'entrance' is the definition.
(type of opening or entrance)
'dockland area with a large' is the wordplay.
'dockland area' becomes 'port' (ports contain docks).
'with' means one lot of letters go next to another.
'large' becomes 'L' (eg in clothes sizes).
'port'+'a'+'l'='PORTAL'
(Other definitions for portal that I've seen before include "An imposing gateway" , "An imposing door or gate" , "Grand and imposing entrance" , "Special website bringing together information from diverse sources in a uniform way" , "'Doorway, entrance (6)'" .)