Bad liar, this bird? (4)
I believe the answer is:
rail
'bird?' is the definition.
(a wading bird)
'bad liar' is the wordplay.
'bad' is an anagram indicator.
'liar' with letters rearranged gives 'RAIL'.
'this' acts as a link.
(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 --" .)