Clue: I've strict diet for one apparently leading pack? (5)
Answer: VEGAN
Explanation: Definition: strict diet
Over to you: I toyed with I'VE GANG, but I cannot quite make it work. *Thanks Flavia @1; it is an implied envelope (which I ought to have seen, having blogged a similar one in the Imogen last week) of EG ('for one') IN VAN ('apparently leading pack'). I think that is much preferable to the second thoughts of Flavia @2, with 'pack' as the envelope indicator, which has syntax problems (the adjectival 'leading' and 'pack' itself). Curiously, the correct parsing came to me out of nowhere at an ungodly hour this morning.