Subject - one included by leading Conservative (5)
I believe the answer is:
topic
'subject' is the definition.
(as in a topic for discussion)
'one included by leading conservative' is the wordplay.
'one' becomes 'I' (Roman numeral).
'included by' indicates putting letters inside.
'leading' becomes 'top' (both can mean first-class).
'conservative' becomes 'C' (UK political abbreviation).
'top'+'c'='topc'
'i' placed into 'topc' is 'TOPIC'.
(Other definitions for topic that I've seen before include "Content of argument" , "Subject (eg, of text)" , "Subject (of speech, etc)" , "Subject matter, theme" , "Subject (for conversation, etc)" .)