Branch of maths involving a bit of geometry - area, length and breadth (7)
I believe the answer is:
algebra
'branch of maths' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
(Other definitions for algebra that I've seen before include "letters here set out the problem" , "Boolean speciality is raised" , "Symbolic mathematics" , "school subject" , "A garble (anag) - branch of mathematics" .)