Raced with vigour round back of course (5)
I believe the answer is:
hared
'raced' is the definition.
(I've seen this in another clue)
'with vigour round back of course' is the wordplay.
'with vigour' becomes 'hard' (hard can mean vigorously or strenuously).
'round' is an insertion indicator.
'back of' says to take the final letters.
The final letter of 'course' is 'e'.
'hard' placed around 'e' is 'HARED'.
(Other definitions for hared that I've seen before include "speed" , "Ran very fast, raced" .)