At school, perhaps, an extract of buttermilk (4)
I believe the answer is:
term
'at school' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'an extract of buttermilk' is the wordplay.
'an extract of' indicates a hidden word (I've seen this in other clues).
'TERM' is hidden in the letters of 'buttermilk'.
'perhaps' is the link.
Can you help me to learn more?
(Other definitions for term that I've seen before include "An expression, or college period" , "Time in jail" , "Limit - school time" , "Part of the school year" , "Period of school life" .)