One following strict diet, say, in the lead (5)
I believe the answer is:
Here is my best explanation:
'one following strict diet' is the definition.
'say in the lead' is the wordplay.
'say' becomes 'eg'.
'in the' means one lot of letters goes inside another.
'lead' becomes 'van' (I've seen this before).
'eg' going within 'van' is 'VEGAN'.