Got in more rubbish in essay subject (12)
I believe the answer is:
trigonometry
'subject' is the definition.
(trigonometry is a kind of subject)
'got in more rubbish in essay' is the wordplay.
'rubbish' is an anagram indicator.
'in' is an insertion indicator.
'essay' becomes 'try' ('try' can be a synonym of 'essay').
'got'+'in'+'more'='gotinmore'
'gotinmore' with letters rearranged gives 'rigonomet'.
'rigonomet' going inside 'try' is 'TRIGONOMETRY'.
(Other definitions for trigonometry that I've seen before include "mathematical skill" , "Branch of mathematics" , "degree-level study" , "Branch of maths" , "Mathematics of the relationship between angles and sides of triangles" .)