Sick bird, we hear, is unlawful (7)
I believe the answer is:
illegal
'is unlawful' is the definition.
(associated in meaning)
'sick bird' is the wordplay.
'sick' becomes 'ill' (synonyms).
'bird' becomes 'egal' (I can't explain this - if you can you should believe this answer much more).
'ill'+'egal'='ILLEGAL'
'we hear' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for illegal that I've seen before include "Contraband" , "Against the rules" , "crooked" , "murder sure is!" , "Forbidden" .)