It's surprising I don't know what leaves eater gutted (5)
I believe the answer is:
corer
'what leaves eater gutted' is the definition.
(a corer might gut a cooking apple - ie an 'eater')
'it's surprising i don't know' is the wordplay.
'it's surprising' becomes 'cor' ('cor' is an expression of surprise).
'i don't know' becomes 'er' ('er' is a hesitation).
'cor'+'er'='CORER'
(Other definitions for corer that I've seen before include "kitchen implement?" , "Device used on apples" , "(Eg, apple) cutter" .)