Sand layer from ancient city covered by modern one? (3,6)
I believe the answer is:
sea turtle
'sand layer' is the definition.
(animal which lays in sand)
'ancient city covered by modern one?' is the wordplay.
'ancient city' becomes 'ur'.
'covered by' indicates putting letters inside.
'modern one?' becomes 'seattle' (I can't justify this - if you can you should give a lot more credence to this answer).
'ur' going inside 'seattle' is 'SEA-TURTLE'.
'from' is the link.
(Other definitions for sea turtle that I've seen before include "Steal true (anag) - marine reptile" , "marine creature" .)