Dieticians: 'Is All-Bran partly fibre?' (5)
I believe the answer is:
sisal
'fibre?' is the definition.
(sisal is a kind of fibre)
'dieticians is all-bran partly' is the wordplay.
'partly' indicates a hidden word.
'SISAL' is hidden in the letters of 'dieticians is all bran'.
(Other definitions for sisal that I've seen before include "constituent of rope" , "Plant fibre used to make string" , "Strong rope fibre" , "Sails (anag) - fibre" , "Agave cultivated for its fibre" .)