My sonny's ill — tired and worn out, maybe (8)
I believe the answer is:
synonyms
'tired and worn out maybe' is the definition.
('tired' and 'worn out' are synonyms)
'my sonny's ill' is the wordplay.
'ill' is an anagram indicator.
'my'+'sonnys'='mysonnys'
'mysonnys' with letters rearranged gives 'SYNONYMS'.
(Other definitions for synonyms that I've seen before include "Two words with the same meaning" , "Words that can be interchanged with others" , "Interchangeable words" , "words such as faith, religion and belief" , "Two words that can be interchanged" .)