'Ratio of a right angle bearing' — jargon! (6)
I believe the answer is:
secant
'ratio of a right angle' is the definition.
(mathematical term)
'bearing jargon' is the wordplay.
'bearing' becomes 'se' (south east, a compass bearing).
'jargon' becomes 'cant' (synonyms).
'se'+'cant'='SECANT'
(Other definitions for secant that I've seen before include "Ratio of the hypotenuse to the adjacent side in a right-angled triangle" , "triangular relationship" , "Trig function" , "Line that intercepts a curve" , "Cutting line (geometry)" .)