Sonny improperly starts to yawn, mocking sir's airs and graces perhaps (8)
I believe the answer is:
synonyms
'airs and graces perhaps' is the definition.
('airs' and 'graces' are synonyms)
'sonny improperly starts to yawn mocking sir's' is the wordplay.
'improperly' is an anagram indicator (I've seen this in other clues).
'starts to' indicates taking the first letters.
The initial letters of 'yawn mocking sirs' is 'yms'.
'sonny' with letters rearranged gives 'synon'.
'synon'+'yms'='SYNONYMS'
(Other definitions for synonyms that I've seen before include "words such as faith, religion and belief" , "Words that can be interchanged with others" , "Interchangeable words" , "Words with meanings similar to those of others" , "Two words with the same meaning" .)