The main idea of work perhaps (5)
I believe the answer is:
theme
This could be a double definition.
'the main idea' is the first definition.
(I've seen this in another clue)
'work perhaps' is the second definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'of' is the link.
Can you help me to learn more?
(Other definitions for theme that I've seen before include "Pervading idea in art" , "Topical item" , "Motif or melody" , "Main idea or topic" , "[(type of) PARK]" .)