The serious student is stony, it seems (6)
I believe the answer is:
gravel
'stony it seems' is the definition.
I can't judge whether this definition defines the answer.
'the serious student' is the wordplay.
'the serious' becomes 'grave' (I've seen this before).
'student' becomes 'l' (as in L-plates for learner drivers**).
'grave'+'l'='GRAVEL'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for gravel that I've seen before include "Mass of small stones" , "Stone mixture for paths" , "Small fragments of rock" , "Rocky fragments" , "Small stones on paths" .)