Of digestion (6)
I believe the answer is:
peptic
'of digestion' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for peptic that I've seen before include "Of digestion, and a related ulcer" , "Kind of ulcer in stomech or duodenum" , "Relating to digestion - kind of ulcer" , "of the digestive system" , "may be aiding digestion" .)