Student pursues trendy antiques, once treated as unimportant (15)
I believe the answer is:
inconsequential
'unimportant' is the definition.
(similar in meaning)
'student pursues trendy antiques once treated' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'pursues' says to put letters next to each other.
'trendy' becomes 'in'.
'treated' indicates an anagram.
'antiques'+'once'='antiquesonce'
'antiquesonce' anagrammed gives 'consequentia'.
'in'+'consequentia'='inconsequentia'
'l' put after 'inconsequentia' is 'INCONSEQUENTIAL'.
'as' acts as a link.
(Other definitions for inconsequential that I've seen before include "Trivial" , "Petty" , "unrelated to the foregoing" .)