I teach trim jockeys some maths (10)
I believe the answer is:
arithmetic
'some maths' is the definition.
(arithmetic is a kind of maths)
'i teach trim jockeys' is the wordplay.
'jockeys' indicates an anagram.
'i'+'teach'+'trim'='iteachtrim'
'iteachtrim' is an anagram of 'ARITHMETIC'.
(Other definitions for arithmetic that I've seen before include "Numerical calculations" , "Branch of mathematics" , "School subject" , "Branch of maths" , "One of the three R's" .)