Bird, a ton in stomach (5)
I believe the answer is:
macaw
'bird' is the definition.
(type of parrot)
'a ton in stomach' is the wordplay.
'ton' becomes 'c'.
'in' indicates putting letters inside.
'stomach' becomes 'maw' (I've seen this before).
'a'+'c'='ac'
'ac' inserted into 'maw' is 'MACAW'.
(Other definitions for macaw that I've seen before include "Chattering cousin of a cockatoo" , "Avian chatterbox" , "Colourful creature" , "Large and showy Latin American parrot" , "Type of parrot" .)