Trading apples has an opening (3)
I believe the answer is:
gap
'an opening' is the definition.
(I know that opening can be written as gap)
'trading apples has' is the wordplay.
'has' indicates a hidden word.
'GAP' is hidden in the letters of 'trading apples'.
(Other definitions for gap that I've seen before include "prospective students may enjoy this year?" , "Break, hole, space" , "Aperture" , "Disparity" , "Fissure" .)