Subject: best one, first in class (5)
I believe the answer is:
topic
'subject' is the definition.
('topic' can be a synonym of 'subject')
'best one first in class' is the wordplay.
'best' becomes 'top' ('top' can be similar in meaning to 'best').
'one' becomes 'i' (Roman numeral).
'first in' suggests taking the first letters.
The initial letter of 'class' is 'c'.
'top'+'i'+'c'='TOPIC'
(Other definitions for topic that I've seen before include "Matter for an argument" , "Subject (eg, of text)" , "Subject (of, eg, speech)" , "Theme of discourse" , "Subject (of a talk, etc)" .)