The animal to put 1lb on (4)
I believe the answer is:
lion
'the animal' is the definition.
(I've seen this before)
'1lb on' is the wordplay.
'llb' becomes 'li' (I can't justify this - if you can you should believe this answer much more).
'li'+'on'='LION'
'to put' is the link.
Can you help me to learn more?
(Other definitions for lion that I've seen before include "Maned beast" , "Large cat; rugby player" , "Buddhist scripture; Sanskrit rule" , "Leo" , "cowardly character in Oz" .)