Bit of maths going over height and circumference (5)
I believe the answer is:
girth
'circumference' is the definition.
(girth is a kind of circumference)
'bit of maths going over height' is the wordplay.
'bit of maths' becomes 'girt' (I can't explain this - if you can you should give a lot more credence to this answer).
'going over' is a charade indicator (letters next to each other).
'height' becomes 'h' (abbreviation in geometry).
'girt'+'h'='GIRTH'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for girth that I've seen before include "Circumferential measure" , "Distance around, often of the tummy" , "Size of waist perhaps" , "Distance round a body" , "Personal circumference" .)