Deer are imitating cattle all around London (5)
I believe the answer is:
moose
'deer' is the definition.
(moose is a kind of deer)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for moose that I've seen before include "North American elk" , "Animal" , "North American deer" , "Ungulate" , "Large northern deer" .)