Block terribly vain student (5)
I believe the answer is:
anvil
'block' is the definition.
(I know that anvil is a type of block)
'terribly vain student' is the wordplay.
'terribly' indicates an anagram.
'student' becomes 'l' (as in L-plates for learner drivers).
'vain' with letters rearranged gives 'anvi'.
'anvi'+'l'='ANVIL'
(Other definitions for anvil that I've seen before include "Iron block; incus" , "Block for metalworking" , "One getting hammered regularly" , "Heavy metal block in forge" , "Heavy block on which to hammer metal" .)