Will ne’er forsake or injure you;

My SECOND, coming from the street

You often trample under feet;

My THIRD you sleep on every night,

Serene and calm, without affright;

My WHOLE is what you should not be

When talking with your friends or me.

[Answer]

372.

My FIRST is a little river in England that gave name to a celebrated university; my SECOND is always near; my THIRD sounds like several large bodies of water; and my WHOLE is the name of a Persian monarch, the neighing of whose horse gave him a kingdom and a crown.