Like rock formed from lava that’s cooled (as in cold) (8)
I believe the answer is:
basaltic
'like rock formed from lava that's cooled' is the definition.
'as in cold' is the wordplay.
'in' is an insertion indicator.
'cold' becomes 'baltic' (baltic can informally mean extremely cold).
'as' placed into 'baltic' is 'BASALTIC'.
(Other definitions for basaltic that I've seen before include "Like black rock" , "Like igneous rock" .)