Time chart I organised for school subject (10)
I believe the answer is:
arithmetic
'school subject' is the definition.
(I've seen this before)
'time chart i organised' is the wordplay.
'organised' indicates anagramming the letters.
'time'+'chart'+'i'='timecharti'
'timecharti' anagrammed gives 'ARITHMETIC'.
'for' is the link.
(Other definitions for arithmetic that I've seen before include "Calculation" , "School subject" , "Numerical calculations" , "Numerical maths" , "summing up maybe" .)