Hard branch of maths? On reflection, it's the circumference (5)
I believe the answer is:
girth
'the circumference' is the definition.
(girth is a kind of circumference)
'hard branch of maths? on reflection' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'branch of maths?' becomes 'trig' (I've seen this before).
'on reflection' says the letters should be written in reverse.
'h'+'trig'='htrig'
'htrig' reversed gives 'GIRTH'.
'it's' is the link.
(Other definitions for girth that I've seen before include "Measure all round" , "Band around a horse's belly" , "Circumpherence" , "all the way round" , "weighty indication?" .)