Bad liar, this bird? (4)
I believe the answer is:
rail
'this bird?' is the definition.
(a wading bird)
'bad liar' is the wordplay.
'bad' indicates anagramming the letters.
'liar' anagrammed gives 'RAIL'.
(Other definitions for rail that I've seen before include "Bar of track or fence" , "Complain bitterly - wading bird" , "Complain intemperately" , "Kind of transport" , "British --; picture --" .)