Father taken aback by delicate stomach (8)
I believe the answer is:
'stomach' is the definition.
(I've seen this before)
'father taken aback by delicate' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'taken aback' says the letters should be written in reverse.
'by' says to put letters next to each other.
'delicate' becomes 'petite' (petite can mean small and delicate).
'pa' backwards is 'ap'.
(Other definitions for appetite that I've seen before include "Desire or liking, for power or food" , "enthusiasm" , "'Strong desire, craving (8)'" , "A feeling of craving something" , "stomach" .)