Triangle ratio cannot get worse (9)
I believe the answer is:
cotangent
'triangle ratio' is the definition.
'cotangent' can be an answer for 'ratio' (I have seen 'Mathematical ratio' mean 'cotangent' so perhaps 'ratio' could also mean 'cotangent'). I am not certain of the 'triangle' bit.
'cannot get worse' is the wordplay.
'worse' indicates anagramming the letters (I've seen this in other clues).
'cannot'+'get'='cannotget'
'cannotget' with letters rearranged gives 'COTANGENT'.
Can you help me to learn more?
(Other definitions for cotangent that I've seen before include "Mathematical ratio" , "Maths function" , "proportion" , "Mathematician's function" , "In triangular relationship" .)