Told popular editor about class (8)
I believe the answer is:
informed
'told' is the definition.
(I've seen this before)
'popular editor about class' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'editor' becomes 'ed' (abbreviation).
'about' is a charade indicator (letters next to each other).
'class' becomes 'form' (form class is a kind of class).
'ed' after 'form' is 'formed'.
'in'+'formed'='INFORMED'
(Other definitions for informed that I've seen before include "Imparted knowledge" , "Knowledgeable or briefed" , "Expert" , "Gave knowledge to" , "Passed on knowledge" .)