I've strict diet for one apparently leading pack? (5)
I believe the answer is:
vegan
'i've strict diet' is the definition.
(vegans have a strict diet)
'one apparently leading pack?' is the wordplay.
I cannot really see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is present in the answer.
The remaining letters 'veg' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'for' acts as a link.
(Other definitions for vegan that I've seen before include "Non-consumer of animal products" , "Strict vegetarian" , "one not using animals" , "Person refraining from using animal products in any way" , "One avoiding all animal-based food" .)