One following strict diet, say, in the lead (5)
I believe the answer is:
vegan
'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'.
(Other definitions for vegan that I've seen before include "one not using animals" , "Non-consumer of animal products" , "Person refraining from eating animal products" , "Person neither eating nor using anything animal" , "Person refraining from using animal products in any way" .)