Trigonometry about primarily hypotenuse and circumference (5)
I believe the answer is:
girth
'circumference' is the definition.
(girth is a kind of circumference)
'trigonometry about primarily hypotenuse' is the wordplay.
'trigonometry' becomes 'trig' ('trig' can be a synonym of 'trigonometry').
'about' says the letters should be written backwards.
'primarily' suggests taking the first letters.
The first letter of 'hypotenuse' is 'h'.
'trig' written backwards gives 'girt'.
'girt'+'h'='GIRTH'
'and' acts as a link.
(Other definitions for girth that I've seen before include "Some tack" , "Distance around, often of the tummy" , "weighty indication?" , "Bulk" , "A band passing under horse's belly" .)