Tuck into apple: a treat! (5)
I believe the answer is:
pleat
'tuck' is the definition.
(I've seen this before)
'into apple a treat' is the wordplay.
'into' says the answer is hidden in the clue.
'PLEAT' is hidden in the letters of 'apple a treat'.
(Other definitions for pleat that I've seen before include "Tuck" , "Gathering" , "that's tailor-made" , "Crease" , "Fold formed in fabric" .)