Fibre in diets is a laxative (5)
I believe the answer is:
sisal
'fibre' is the definition.
(sisal is a kind of fibre)
'in diets is a laxative' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SISAL' is hidden in the letters of 'diets is a laxative'.
(Other definitions for sisal that I've seen before include "Agave cultivated for its fibre" , "Rope fibre from leaves of tropical plant" , "Agave yielding fibre" , "Agave rope-fibre" , "Fibre used in rope-making" .)