Consequences? Not after a lesson at school (5)
I believe the answer is:
maths
'school' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'consequences? not after a lesson' is the wordplay.
'consequences?' becomes 'aftermath' (the aftermath of something is its consequences).
'not' is a deletion indicator.
'a lesson' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'aftermath' with 'after' removed is 'math'.
'math'+'s'='MATHS'
'at' acts as a link.
Can you help me to learn more?
(Other definitions for maths that I've seen before include "Arithmetic etc" , "Subject involving figures (abbr)" , "Numbers subject" , "18 across subject" , "Study of numbers (abbr.)" .)