Not a subject put on in advance (7)
I believe the answer is:
monarch
'not a subject' is the definition.
(I've seen this before)
'on in advance' is the wordplay.
'in' is an insertion indicator.
'advance' becomes 'march' (march is a kind of advance).
'on' placed within 'march' is 'MONARCH'.
'put' acts as a link.
(Other definitions for monarch that I've seen before include "Butterfly (emperor, say?)" , "Butterfly - ruler" , "one over realm?" , "Sovereign; butterfly" , "Sole hereditary ruler" .)