Monk astride large beast (5)
I believe the answer is:
llama
'beast' is the definition.
(I've seen this before)
'monk astride large' is the wordplay.
'monk' becomes 'lama' (as in the Dalai Lama).
'astride' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'lama' enclosing 'l' is 'LLAMA'.
(Other definitions for llama that I've seen before include "Alpaca" , "Andean pack animal" , "South American cud-chewing mammal" , "Even-toed camel relative" , "20vian beast" .)