Crucial-sounding place for loading and unloading ships (4)
I believe the answer is:
quay
'place for loading and unloading ships' is the definition.
'crucial-sounding' is the wordplay.
'crucial' becomes 'key' (I've seen this before).
'sounding' shows a homophone (sound like).
'key' is a homophone of 'QUAY'.
(Other definitions for quay that I've seen before include "Dock's loading area" , "Landing-place" , "crucial, by the sound of it" , "Jetty" , "Whatf built parallel to the shoreline" .)