Occupying position of king in charge of subject (5)
I believe the answer is:
topic
'subject' is the definition.
(I know that subject can be written as topic)
'occupying position of king in charge' is the wordplay.
'occupying position of king' becomes 'top' (the king is at the top).
'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 "Subject (of, eg, speech)" , "Question" , "Subject (of speech, etc)" , "one discussed" , "Matter for an argument" .)