Bird, rook, to be ill and weak (4)
I believe the answer is:
rail
'bird' is the definition.
(a wading bird)
'rook to be ill and weak' is the wordplay.
I cannot quite understand how this works, but
'rook' could be 'r' (chess abbreviation) and 'r' is located in the answer.
'be ill' could be 'ail' (synonyms) and 'ail' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rail that I've seen before include "Means of transport" , "method of transport" , "Bar of track or fence" , "Form of transport" , "The permanent way" .)