Dull student in the group (5)
I believe the answer is:
bland
'dull' is the definition.
(similar in meaning)
'student in the group' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'in the' is an insertion indicator.
'group' becomes 'band' (band can mean a group of people).
'l' inserted inside 'band' is 'BLAND'.
(Other definitions for bland that I've seen before include "Dull - tasteless" , "Insipid - uninteresting" , "Boring" , "Mild; dull" , "Suave" .)