Vegetable eating wholly brought about King George's disease (8)
I believe the answer is:
pellagra
'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'.
'lla'+'gr'='llagr'
'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" .)