Area of interest to film (5)
I believe the answer is:
topic
'area of interest' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'to film' is the wordplay.
'film' 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 "'Theme, subject (5)'" , "A subject for conversation or debate" , "Subject matter, theme" , "Theme of discourse" , "Talking point" .)