Glam rockers back at top of class; we're at back! (7)
I believe the answer is:
dumbest
'class we're at back' is the definition.
I can't tell whether this definition defines the answer.
'glam rockers back at top' is the wordplay.
I cannot quite see how this works, but
'top' could be 'best' ('best' can be similar in meaning to 'top') and 'best' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for dumbest that I've seen before include "Very stupid" , "Thickest" .)