Pip found in apple, not spinach, on reflection (5)
I believe the answer is:
stone
'pip' is the definition.
(I've seen this before)
'in apple not spinach on reflection' is the wordplay.
'in' indicates a hidden word.
'on reflection' is a reversal indicator.
'enots' is hidden in the letters of 'apple not spinach'.
'enots' back-to-front is 'STONE'.
'found' acts as a link.
(Other definitions for stone that I've seen before include "Gem; Staffs. town" , "Boulder" , "Rag" , "Rock; gem" , "See 21" .)