Vegetable eating wholly brought about King George's disease (8)
I believe the answer is:
'disease' is the definition.
(pellagra is a kind of disease)
'vegetable eating wholly brought about king george's' is the wordplay.
'vegetable' becomes 'pea' (pea is a kind of vegetable).
'eating' is an insertion indicator.
'wholly' becomes 'all' ('all' can be a synonym of 'wholly').
'brought about' is a reversal indicator.
'king george' becomes 'gr' (Georgius Rex).
'all' reversed gives 'lla'.
'pea' placed around 'llagr' is 'PELLAGRA'.
(Other definitions for pellagra that I've seen before include "Wasting disease, caused by a Vitamin B deficiency" , "ill with this?" , "Itching" .)