Rugs take his first calculations (5)
I believe the answer is:
maths
'calculations' is the definition.
The answer and definition can be both related to thinking as well as being plural nouns.
Maybe they are linked in a way I don't understand?
'rugs take his first' is the wordplay.
'rugs' becomes 'mats' (I've seen this before).
'take' indicates putting letters inside.
'his first' becomes 'h' (1st letter of 'his').
'mats' enclosing 'h' is 'MATHS'.
Can you help me to learn more?
(Other definitions for maths that I've seen before include "18 across subject" , "Adding and subtraction, etc" , "Study of numbers and sums" , "A school subject (abbr.)" , "subject, in short" .)