Like rock when submerged by sea (8)
I believe the answer is:
basaltic
This clue might be a double definition.
'like rock' is the first definition.
'submerged by sea' is the second definition.
Although both the answer and definition are adjectives, I cannot understand how they can define each other.
'when' is the link.
(Other definitions for basaltic that I've seen before include "Like black rock" , "Like igneous rock" .)