Like some rocks, since covered by the sea (8)
I believe the answer is:
basaltic
'like some rocks' is the definition.
(basalt is a type of igneous rock)
'since covered by the sea' is the wordplay.
'since' becomes 'as' (I've seen this in other clues).
'covered by' means one lot of letters goes inside another.
'the sea' becomes 'baltic' (Baltic is an example).
'as' put within 'baltic' is 'BASALTIC'.
(Other definitions for basaltic that I've seen before include "Like black rock" , "Like igneous rock" .)