Trigonometrical ratio is able to be seen in series (6)
I believe the answer is:
secant
'trigonometrical ratio' is the definition.
The definition and answer can be both relationships as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'able to be seen in series' is the wordplay.
'able' becomes 'can'.
'to be seen in' indicates putting letters inside.
'series' becomes 'set' (I've seen this before).
'can' put into 'set' is 'SECANT'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for secant that I've seen before include "Line that intercepts a curve" , "Line that intersects a curve at two or more points" , "Cutting line (geometry)" , "triangular relationship" , "Ratio of the hypotenuse to the adjacent side in a right-angled triangle" .)