Pretend to have mounting appetite for offal (6)
I believe the answer is:
kidney
'offal' is the definition.
'pretend to have mounting appetite' is the wordplay.
'pretend' becomes 'kid' (I've seen this before).
'to have' says to put letters next to each other.
'mounting' says the letters should be written in reverse (going up in a down clue).
'appetite' becomes 'yen'.
'yen' backwards is 'ney'.
'kid'+'ney'='KIDNEY'
'for' acts as a link.
(Other definitions for kidney that I've seen before include "body part" , "Character" , "Excreting organ" , "Organ; kind; bean type" , "Bodily organ" .)