Row of bushes giving first of horses advantage (5)
I believe the answer is:
hedge
'row of bushes' is the definition.
(I've seen this in another clue)
'giving first of horses advantage' is the wordplay.
'giving' means one lot of letters go next to another.
'first of horses' becomes 'h' (1st letter of 'horses').
'advantage' becomes 'edge' (having the edge is having an advantage).
'h' next to 'edge' is 'HEDGE'.
(Other definitions for hedge that I've seen before include "take evasive action" , "Row of bushes" , "Be shifty" , "Barrier of shrubs or bushes" , "appear non-committal in statement" .)