A supporter nursing broken leg in class (7)
I believe the answer is:
algebra
'class' is the definition.
(I know that algebra is a class)
'a supporter nursing broken leg' is the wordplay.
'supporter' becomes 'bra' (a bra gives support to breasts).
'nursing' means one lot of letters goes inside another.
'broken' indicates an anagram.
'a'+'bra'='abra'
'leg' is an anagram of 'lge'.
'abra' placed around 'lge' is 'ALGEBRA'.
'in' acts as a link.
(Other definitions for algebra that I've seen before include "system employing symbols" , "Branch of maths using letters and symbols" , "Symbolic mathematics" , "Form of maths" , "letters here set out the problem" .)