Across lake, it's seriously stony (8)
I believe the answer is:
gravelly
'stony' is the definition.
(thesaurus)
'across lake it's seriously' is the wordplay.
'across' is an insertion indicator.
'lake' becomes 'l'.
'it's seriously' becomes 'gravely' ('gravely' can be a synonym of 'seriously').
'l' placed into 'gravely' is 'GRAVELLY'.
(Other definitions for gravelly that I've seen before include "Like a drive" , "(Of voice) deep, rough" , "Harsh and grating (of voice)" .)