Trot like horsie round one's box (8)
I believe the answer is:
canister
'box' is the definition.
(I've seen this before)
'trot like horsie round one's' is the wordplay.
I cannot really understand how this works, but
'one's' could be 'is' (Roman numeral) and 'is' is present in the answer.
The remaining letters 'canter' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for canister that I've seen before include "Metal storage container" , "Metal container for storing dry foods" , "Cylindrical container" , "Tea-caddy" , "Jar (of e.g. tea, sugar)" .)