Shifting sand blocks Greek port (6)
I believe the answer is:
gdansk
'port' is the definition.
(port in Poland)
'shifting sand blocks greek' is the wordplay.
'shifting' is an anagram indicator.
'blocks' indicates putting letters inside (upper walls off glass inside formal).
'greek' becomes 'gk'.
'sand' anagrammed gives 'dans'.
'dans' put inside 'gk' is 'GDANSK'.
(Other definitions for gdansk that I've seen before include "Polish Baltic port" , "Major Polish port" , "It was once called Danzig" , "Polish coastal city" , "Polish town" .)