Picked up a small, headless mammal (5)
I believe the answer is:
llama
'mammal' is the definition.
(llama is a kind of mammal)
'picked up a small headless' is the wordplay.
'picked up' says the letters should be written in reverse.
'headless' means to remove the first letter.
'small' with its first letter taken off is 'mall'.
'a'+'mall'='amall'
'amall' in reverse letter order is 'LLAMA'.
(Other definitions for llama that I've seen before include "20vian beast" , "A native of South America" , "South American transport animal" , "South American relative of the camel" , "As bearer of burdens" .)