Lie about what is in roughage (5)
I believe the answer is:
fibre
'what is in roughage' is the definition.
(roughage means food containing fibre)
'lie about' is the wordplay.
'lie' becomes 'fib' (fib is a kind of lie).
'about' becomes 're' ('re' means regarding or about).
'fib'+'re'='FIBRE'
(Other definitions for fibre that I've seen before include "Coarse plant food, low in nutrients" , "Basis of yarn" , "Texture - strength of character" , "Long fine thread" , "Brief (anag.)" .)