Head in charge of subject (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'
'of' is the link.
(Other definitions for topic that I've seen before include "Matter for an argument" , "Subject matter of a conversation or discussion" , "Subject matter, theme" , "Subject (of a talk, etc)" , "'Theme, subject (5)'" .)