The crowds upon the pavement Were fields of harvest wheat. (3-4)
This metaphor in stanza 1 introduces the poem's Death theme. All those people in the crowd become soon-to-be harvested wheat stalks. Harvested wheat = dead people. It isn't a happy equation, but that's what Auden gives us.
'Into many a green valley Drifts the appalling snow; (33-34)
That green valley represents life. Green is the color of new growth and renewal. Shmoop likes green. Unfortunately, we don't get to enjoy that fertile, vibrant valley very long. In the very next line the "appalling snow" drifts in and, we can assume, kills everything. Score another one for Death. Boo.
And the crack in the tea-cup opens A lane to the land of the dead. (43-44)
These lines are kind of mysterious, but they do make an impression. Auden seems to be suggesting that reminders of death are everywhere. Not just in the big, dramatic things like wars or disasters, but also in the small, everyday examples of wear and tear (like a cracked tea-cup). It all comes back to Time taking its toll on things. In these lines, the clock-speaker is trying to get us to see this. He wants us to equate that crack in the tea-cup with the aging and deterioration of our bodies and minds that will, eventually, lead to death. Thanks a lot for the reminder.