Serious student of stones (6)
I believe the answer is:
gravel
'stones' is the definition.
(I've seen this before)
'serious student' is the wordplay.
'serious' becomes 'grave' (both can mean solemn).
'student' becomes 'l' (as in L-plates for learner drivers).
'grave'+'l'='GRAVEL'
'of' is the link.
(Other definitions for gravel that I've seen before include "bits of rock" , "Shingle" , "Loose fragments of rock" , "Rock fragments and pebbles" , "Small stones for surfacing driveways" .)