Monk astride large beast of burden (5)
I believe the answer is:
llama
'beast of burden' is the definition.
(I've seen this in another clue)
'monk astride large' is the wordplay.
'monk' becomes 'lama' (as in the Dalai Lama).
'astride' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'lama' placed around 'l' is 'LLAMA'.
(Other definitions for llama that I've seen before include "S American animal like a camel with soft hair" , "one to chew the cud" , "S.American animal" , "Long-necked woolly animal" , "woolly customer" .)