Different course brings wild nag into shelter (7)
I believe the answer is:
tangent
'different course' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'wild nag into shelter' is the wordplay.
'wild' is an anagram indicator.
'into' indicates putting letters inside.
'shelter' becomes 'tent' (tent is a kind of shelter).
'nag' with letters rearranged gives 'ang'.
'ang' going inside 'tent' is 'TANGENT'.
'brings' is the link.
Can you help me to learn more?
(Other definitions for tangent that I've seen before include "Straight line just touching a curve" , "Geometric line" , "Line touching circumference" , "Line grazing another" , "mathematician's line" .)