I read in your message that you have never completed an RT crossword. That's a great shame and all these people here aren't helping much are they? Too much Christmas Spirit I expect. Anyway, as it's Christmas I'll be kind and show you the best way to ask for help so that you don't get answers from the likes of Richard Head (AKA "Please call me Dick").
57A Gloomy mob's moving about (6) S?M?R?
and now the answer you've been waiting for:
SOMBRE (gloomy) = SOMB (mob's moving) + RE (about)
There, now you've managed to finish the "Big One". There'll be another RT quite soon so we can start the New Year with #1 and your success still ringing in our ears.