Complain bitterly of a terrible liar (4)
I believe the answer is:
rail
'complain bitterly' is the definition.
(I've seen this in another clue)
'a terrible liar' is the wordplay.
'a terrible' is an anagram indicator.
'liar' is an anagram of 'RAIL'.
'of' acts as a link.
(Other definitions for rail that I've seen before include "Supported bar - inveigh" , "20 backwards, that 7 runs on" , "Use abusive language" , "Complain bitterly - wading bird" , "Means of transport" .)