Litres swallowed by enthusiastic, tailless bird (5)
I believe the answer is:
eagle
'bird' is the definition.
(eagle is a type of bird)
'litres swallowed by enthusiastic tailless' is the wordplay.
'litres' becomes 'l' (abbreviation for litres).
'swallowed by' indicates putting letters inside.
'enthusiastic' becomes 'eager' (similar in meaning).
'tailless' means to remove the last letter (the tail of the word is removed).
'eager' with its final letter taken off is 'eage'.
'l' placed within 'eage' is 'EAGLE'.
(Other definitions for eagle that I've seen before include "Hook-billed bird of prey -- old US $10 coin" , "High-flyer" , "drugs with silver left inside" , "kind of bird" , "Symbolic bird of the US" .)