Chief in charge of subject (5)
I believe the answer is:
topic
'subject' is the definition.
('topic' can be a synonym of 'subject')
'chief in charge' is the wordplay.
'chief' becomes 'top' ('top' can be a synonym of 'chief').
'in charge' becomes 'ic'.
'top'+'ic'='TOPIC'
'of' acts as a link.
(Other definitions for topic that I've seen before include "Subject (of, eg, speech)" , "'Theme, subject (5)'" , "Question" , "A subject for conversation or debate" , "might this be hot?" .)