Smile as you're swallowing a piece of cake (4)
I believe the answer is:
easy
'a piece of cake' is the definition.
(I've seen this before)
'smile as you're swallowing' is the wordplay.
'swallowing' indicates the answer is hidden within the clue.
'EASY' can be found hidden inside 'smile as youre'.
(Other definitions for easy that I've seen before include "Not challenging" , "Undemanding" , "Facile" , "Comfortable" , "Men may be told to stand thus" .)