Refactoring from .. in .. select to more compact one
Idon't see why you would want to shorten that piece of code but here's the same result in one line and using a LinQ method-chain instead of query-syntax:

var response = otherClass.SomeMethod(contract.Contact.PartnerNumbers.Select(p =&gt; p.Pnr), DateTime.Now.Date);


Edit:

Since you have no parameters that have influence on the calculation of the response, here's a method for re-usability:

public int GetResponse()
{
    var collection = from partnerNumbers in contract.Contact.PartnerNumbers select partnerNumbers.Pnr;
    var response = this.otherClass.SomeMethod(collection, DateTime.Now.Date);
    return response;
}


or with LinQ method-chain:

public int GetResponse()
{
    return otherClass.SomeMethod(contract.Contact.PartnerNumbers.Select(p =&gt; p.Pnr), DateTime.Now.Date);
}