Seed to pop back into apple (6)
I believe the answer is:
pippin
'apple' is the definition.
(pippin is a kind of apple)
'seed to pop back into' is the wordplay.
'seed' becomes 'pip' (I've seen this before).
'to' says to put letters next to each other.
'back' suggests the final letters.
'into' becomes 'in'.
The last letter of 'pop' is 'p'.
'pip'+'p'+'in'='PIPPIN'
(Other definitions for pippin that I've seen before include "excellent example" , "Hobbit, friend of 1 across" , "fruit" , "beauty" , "Variety of eating apple" .)