You could just make it not use a let
:
var results = from r in Tickets
where r.Team.Equals("ABC") && r.DateCreated>1330001100 && r.DateCreated<1350000000
group r by r.priority into u
select new
{
Priority = u.Key,
TicketCount = u.Count(),
AverageMin = u.Average(x => x.FinishDate - x.DateCreated)
};