Female friend returned sailor's biscuit (8)
I believe the answer is:
'biscuit' is the definition.
(I've seen this before)
'female friend returned sailor's' is the wordplay.
'female' becomes 'f' (abbreviation).
'friend' becomes 'pal' (pal is a kind of friend**).
'returned' says the letters should be written in reverse.
'sailor' becomes 'jack' (I've seen this before).
'pal' written backwards gives 'lap'.
(Other definitions for flapjack that I've seen before include "Thick oat biscuit" , "Chewy cake with oats and syrup" , "American pancake" , "It's sweet" , "Oaty biscuit; US pancake" .)