I had entered the arena on horseback (6)
I believe the answer is:
riding
'on horseback' is the definition.
(I've seen this before)
'i had entered the arena' is the wordplay.
'i had' becomes 'id' (I'd).
'entered' indicates putting letters inside.
'the arena' becomes 'ring' (I've seen this in other clues).
'id' inserted within 'ring' is 'RIDING'.
(Other definitions for riding that I've seen before include "Going cycling, perhaps" , "being carried" , "Division of Yorkshire" , "Travelling on horseback" , "Sitting on and controlling the movements of a horse" .)