Style, class to a T (6)
I believe the answer is:
format
'style' is the definition.
(I've seen this in another clue)
'class to a t' is the wordplay.
'class' becomes 'form' (form class is a kind of class).
'form'+'a'+'t'='FORMAT'
(Other definitions for format that I've seen before include "Way of presentation" , "Secure firmly in position" , "Way in which something is presented" , "magazine" , "General arrangement of a publication" .)