Look up papers first for sacred object (4)
I believe the answer is:
idol
'sacred object' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'look up papers first' is the wordplay.
'look' becomes 'lo' (archaic exclamation meaning 'look!').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'papers' becomes 'ID' (identifying documents are one's 'papers').
'first' means one lot of letters go next to another.
'lo' back-to-front is 'ol'.
'ol' after 'id' is 'IDOL'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for idol that I've seen before include "Superstar; graven image" , "Heathen god" , "Much-loved actor" , "Admired figure" , "Person who is the object of intense admiration as by fan club" .)