Course in which Pole collects top grade (4)
I believe the answer is:
road
'course' is the definition.
(thesaurus)
'pole collects top grade' is the wordplay.
'pole' becomes 'rod' ('rod' can be a synonym of 'pole').
'collects' means one lot of letters goes inside another.
'top grade' becomes 'a'.
'rod' going around 'a' is 'ROAD'.
'in which' acts as a link.
(Other definitions for road that I've seen before include "See 2" , "'The . . . . to hell is paved with good intentions'" , "course" , "See 10" , "motorway?" .)