Wharf that's most important, we hear (4)
I believe the answer is:
quay
'wharf' is the definition.
(I know that quay is a type of wharf)
'most important we hear' is the wordplay.
'most important' becomes 'key' (I've seen this in another clue).
'we hear' indicates a 'sounds like' (homophone) clue.
'key' is a homophone of 'QUAY'.
'that's' acts as a link.
(Other definitions for quay that I've seen before include "Jetty" , "Dock's loading area" , "Dock, wharf" , "Pier" , "Landing stage, wharf" .)