Source of apple tree to fix product of apple tree (6)
I believe the answer is:
pippin
'product of apple tree' is the definition.
(pippin is a type of apple)
'source of apple tree to fix' is the wordplay.
'source of apple tree' becomes 'pip' (an apple tree grows from a pip).
'to fix' becomes 'pin' (to pin is to fix in place).
'pip'+'pin'='PIPPIN'
(Other definitions for pippin that I've seen before include "excellent example" , "fruit" , "Hobbit, friend of 1 across" , "Variety of eating apple" , "beauty" .)