Trump's description perhaps provides digression (7)
I believe the answer is:
tangent
'trump's' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for tangent that I've seen before include "Straight line touching a circle" , "What people often go off at!" , "At which one can go off" , "Completely different line of thought - trigonometrical function" , "Straight line that touches but does not intersect a circle" .)