Go or walk briskly (4)
I believe the answer is:
'go or walk briskly' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for trot that I've seen before include "Move at brisk pace" , "(Horse's) pace faster than a walk" , "Left wing" , "Communist - horse pace faster that a walk" , "movement of H [TROT]" .)