More or less how old is this dietary fibre? (8)
I believe the answer is:
roughage
'this dietary fibre?' is the definition.
(I've seen this before)
'more or less how old' is the wordplay.
'more or less' becomes 'rough' (as in a rough estimate).
'how old' becomes 'age' (I've seen this before).
'rough'+'age'='ROUGHAGE'
'is' is the link.
(Other definitions for roughage that I've seen before include "It aids digestion" , "Indigestible fibre" , "Vegetable fibre" , "Coarse food" , "something that can make your passing more bearable" .)