Crown in charge of subject (5)
I believe the answer is:
topic
'subject' is the definition.
(I know that subject can be written as topic)
'crown in charge' is the wordplay.
'crown' becomes 'top' (synonyms).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'top'+'ic'='TOPIC'
'of' acts as a link.
(Other definitions for topic that I've seen before include "Content of argument" , "'Theme, subject (5)'" , "Subject (eg, of text)" , "Theme (of, eg, a talk)" , "Something to discuss" .)