Crown's in charge for subject (5)
I believe the answer is:
topic
'subject' is the definition.
('topic' can be a synonym of 'subject')
'crown's in charge' is the wordplay.
'crown' becomes 'top' ('top' can be a synonym of 'crown').
'in charge' becomes 'ic'.
'top'+'ic'='TOPIC'
'for' is the link.
(Other definitions for topic that I've seen before include "Subject (for conversation, etc)" , "Content of argument" , "Question" , "Subject (of speech, etc)" , "A subject for conversation or debate" .)