First chapter on one subject (5)
I believe the answer is:
topic
'subject' is the definition.
(synonyms)
'first chapter on one' is the wordplay.
'first' becomes 'top' (synonyms).
'chapter' becomes 'c'.
'on' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'c' put after 'i' is 'ic'.
'top'+'ic'='TOPIC'
(Other definitions for topic that I've seen before include "Matter for an argument" , "Subject matter, theme" , "Subject matter of a conversation or discussion" , "Theme (of, eg, a talk)" , "Subject (for conversation, etc)" .)