Subject head in charge (5)
I believe the answer is:
topic
'subject' is the definition.
(as in a topic for discussion)
'head in charge' is the wordplay.
'head' becomes 'top' (synonyms).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'top'+'ic'='TOPIC'
(Other definitions for topic that I've seen before include "Theme of discourse" , "Something to discuss" , "That may be debatable" , "Subject (of speech, etc)" , "Issue (for discussion)" .)