Promoter of diet, say, in the forefront (5)
I believe the answer is:
vegan
'promoter of diet' is the definition.
'say in the forefront' is the wordplay.
'say' becomes 'eg'.
'in the' means one lot of letters goes inside another.
'forefront' becomes 'van' (I've seen this before).
'eg' going inside 'van' is 'VEGAN'.
(Other definitions for vegan that I've seen before include "One shunning animal products" , "Person neither eating nor using anything animal" , "Avoiding animal products" , "Selective consumer" , "Avoider of animal products" .)