Man possibly digesting core of dietary fibre (5)
I believe the answer is:
istle
'fibre' is the definition.
(I've seen this before)
'man possibly digesting core of dietary' is the wordplay.
'man possibly' becomes 'isle' (Isle of Man is an example).
'digesting' is an insertion indicator (inserted letters are eaten or ingested).
'core of' indicates the central letters.
The middle of 'dietary' is 't'.
'isle' placed around 't' is 'ISTLE'.
(Other definitions for istle that I've seen before include "for making nets?" , "Fibre from yucca or agave trees" , "Carpet material" .)