Untrue statement about an essential part of one's diet (5)
I believe the answer is:
fibre
'an essential part of one's diet' is the definition.
I can't judge whether this definition defines the answer.
'untrue statement about' is the wordplay.
'untrue statement' becomes 'fib' (fib is a kind of statement. I am not sure about the 'untrue' bit.).
'about' becomes 're' ('re' means regarding or about).
'fib'+'re'='FIBRE'
Can you help me to learn more?
(Other definitions for fibre that I've seen before include "Dietary material; thread" , "(Material composed of) thread(s)" , "In brief, a thread" , "Threadlike substance" , "Coarse plant food, low in nutrients" .)