With a broken leg support branch of mathematics? (7)
I believe the answer is:
algebra
'branch of mathematics?' is the definition.
(I've seen this before)
'with a broken leg support' is the wordplay.
'with' is an insertion indicator.
'broken' indicates anagramming the letters.
'support' becomes 'bra' (a support for the breasts).
'leg' anagrammed gives 'lge'.
'a' put within 'lge' is 'alge'.
'alge'+'bra'='ALGEBRA'
(Other definitions for algebra that I've seen before include "letters here set out the problem" , "for study in class" , "Branch of mathematics using letters and symbols" , "Boolean speciality is raised" , "Branch of generalised arithmetic" .)