Son finishing off dull hour in school lesson (5)
I believe the answer is:
maths
'school lesson' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'son finishing off dull hour' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'finishing off' says to put letters next to each other (some letters finish or go after others).
'dull' becomes 'mat' (mat as opposed to glossy).
'hour' becomes 'h' (abbreviation).
'mat'+'h'='math'
's' after 'math' is 'MATHS'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for maths that I've seen before include "Study of numbers (abbr.)" , "A school subject (abbr.)" , "Adding and subtraction, etc" , "Science dealing with quantity and shapes, in short" , "Subject involving figures (abbr)" .)