Set subject to photograph (5)
I believe the answer is:
topic
'set subject' is the definition.
'topic' can be an answer for 'subject' (as in a topic for discussion). I am not sure about the 'set' bit.
'to photograph' is the wordplay.
'photograph' becomes 'pic' (short for picture).
'to'+'pic'='TOPIC'
Can you help me to learn more?
(Other definitions for topic that I've seen before include "Issue (for discussion)" , "'Theme, subject (5)'" , "Subject matter of a conversation or discussion" , "Subject (of a talk, etc)" , "Something to discuss" .)