Subject of fiction, say, written up (5)
I believe the answer is:
liege
'subject' is the definition.
(I've seen this before)
'fiction say written up' is the wordplay.
'fiction' becomes 'lie' (fiction can mean a lie or fabrication).
'say' becomes 'eg' (both can mean 'for example').
'written up' is a reversal indicator (in a down clues, letters go up).
'eg' in reverse letter order is 'ge'.
'lie'+'ge'='LIEGE'
'of' is the link.
(Other definitions for liege that I've seen before include "Belgian city - master - servant" , "Vassal; city in Belgium" , "Belgian city or feudal superior" , "City in eastern Belgium" , "Bound to render feudal service" .)