By far the quickest way is going to be not to go to clapham junction but to go to clapham common and walk the 10 minutes from there, one tube change, shouldn't be too bad though you'll need to leave early.
In fact, I was out in hillingdon a few weeks back and it took best part of an hour to get back to the city by tube, given that this is further, first train will be no earlier than 5-30 (I don't know exactly on the metropolitan line), I think you'll struggle, if you need to make it get a taxi.