First-class seminar on one chapter (7)
I believe the answer is:
classic
'first-class' is the definition.
(I've seen this before)
'seminar on one chapter' is the wordplay.
'seminar' becomes 'class' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'one' becomes 'I' (Roman numeral).
'chapter' becomes 'c' (abbreviation).
'class'+'i'+'c'='CLASSIC'
(Other definitions for classic that I've seen before include "Oaks perhaps" , "work of the highest quality" , "lasting literary work" , "Time-honoured" , "Very great work of art" .)