Never-ending trouble for a bird (4)
I believe the answer is:
rail
'bird' is the definition.
(a wading bird)
'never-ending trouble' is the wordplay.
'ending' indicates one should take the final letters (the ending of the word).
'trouble' becomes 'ail' (synonyms).
The final letter of 'never' is 'r'.
'r'+'ail'='RAIL'
'for a' acts as a link.
(Other definitions for rail that I've seen before include "Scold, berate" , "Complain vehemently" , "Line" , "Safety bar; bird" , "Abuse, scold" .)