Don: boy biting cheek (4,2)
I believe the answer is:
slip on
'don' is the definition.
(I know that slip on is a type of don)
'boy biting cheek' is the wordplay.
'boy' becomes 'Son' (synonyms).
'biting' indicates putting letters inside (some letters bite or swallow others).
'cheek' becomes 'lip' (both can mean insolence or rudeness).
'son' placed around 'lip' is 'SLIP-ON'.
(Other definitions for slip on that I've seen before include "Type of garment" , "shoe, perhaps" , "easy to cope with when dressing" .)