Part of circular course? (3)
I believe the answer is:
arc
'part' is the definition.
(I have seen 'Part of circumference' mean 'arc' so perhaps 'part' could also mean 'arc')
'of circular course?' is the wordplay.
'of' says the answer is hidden in the clue.
'ARC' can be found hidden inside 'circular course'.
(Other definitions for arc that I've seen before include "development of plot" , "less than the complete circuit" , "Semi-circle" , "Portion of a circle" , "Luminous electrical discharge" .)