Question

How do I declare such a variable?

            var rData = from nc in ctx.NEWSLETTER_CLIENTS
                        join ni in ctx.NEWSLETTER_INDICES on nc.INDEX_NUM 
                                                          equals ni.INDEX_NUM
                        select new
                        {
                            ClientID = nc.CLIENT_ID,
                            Email = nc.CLIENT_EMAIL_ADDRESS,
                            Index = nc.INDEX_NUM,
                            MainClass = ni.MAIN_CLASS,
                            SubClass = ni.SUB_CLASS,
                            App1 = ni.VALUE_1,
                            App2 = ni.VALUE_2,
                            App3 = ni.VALUE_3,
                            App4 = ni.VALUE_4
                        };

        // Now I need to declare on a variable named fData under the function scope,
        // so I can later use it:

        var fData = ...; //What do I declare here?

        if(x)
            fData = fData.Concat(rData.Where(u => ...));
        if(y)
            fData = fData.Concat(rData.Where(u => ...));
        // etc

Answer 1

IQueryable<type of p> fData = null;

If you want to use the query later (iow after the if):

var fData = Enumerable.Empty<type of p>().AsQueryable();

Update:

Now for using with anonymous types:

IQueryable<T> RestOfMethod<T>(IQueryable<T> rData)
{
  var fData = Enumerable.Empty<T>().AsQueryable(); // or = rData;

  if(x)
    fData = fData.Concat(rData.Where(u => ...));
  if(y)
    fData = fData.Concat(rData.Where(u => ...));

  return fData;
}

// original code location
var rData = some query;
var fData = RestOfMethod(rData);

Update 2:

As pointed out, the above does not actually work, as the predicate of Where does not know the type. You could refactor it some more to include the predicates in the parameters, example:

IQueryable<T> RestOfMethod<T>(IQueryable<T> rData, 
  Expression<Func<T,bool>> pred1,
  Expression<Func<T,bool>> pred2) 
{ ... }

Update 3: (perhaps hacky)

var fData = rData.Take(0); // should be cheap.