Subject that’s closed, still? (5)
I believe the answer is:
topic
'subject' is the definition.
(as in a topic for discussion)
'closed still?' is the wordplay.
'closed' becomes 'to' (as in a door closed to).
'still?' becomes 'pic' (short for picture).
'to'+'pic'='TOPIC'
'that's' acts as a link.
(Other definitions for topic that I've seen before include "Subject matter of a conversation or discussion" , "one discussed" , "Content of argument" , "Matter for an argument" , "Question" .)