Landing-place is crucial, we hear (4)
I believe the answer is:
quay
'landing-place' is the definition.
(I've seen this before)
'crucial we hear' is the wordplay.
'crucial' becomes 'key' (I've seen this before).
'we hear' shows a homophone (sound like).
'key' sounds like 'QUAY'.
'is' acts as a link.
(Other definitions for quay that I've seen before include "Landing place" , "Some of port" , "Dock's loading area" , "Landing area" , "Pier" .)