UNDER is a one-sided word. It heads 12 compounds in this list and finishes none of them, so it can start a chain but never sit in the middle of one.
Each of these makes a compound starting with UNDER.
UNDER heads twelve compounds here and finishes none. Most are literal — UNDERGROUND, UNDERWATER, UNDERFOOT — but UNDERSTAND has nothing beneath anything: its under may once have meant between or among rather than below, so to understand something was to stand in the middle of it.
KEYSTONE is a free daily puzzle built entirely out of chains like these. You get the two ends and have to fill the two words in the middle. Three bridges a day, no account, no cookies.
Each of these has a page like this one. There is also an index of all 54.
AWAY · BACK · BALL · BED · BLACK · BOARD · BOOK · BOY · BREAK · CHECK · DAY · DOOR · DOWN · FALL · FATHER · FIRE · GROUND · GUN · HAND · HEAD · HOOD · HOUSE · LAND · LESS · LIFE · LIGHT · LINE · LOAD · MAN · MOTHER · NIGHT · OFF · OUT · OVER · PAY · POINT · POST · POWER · ROOM · RUN · SHIP · SIDE · SOME · STEP · SUN · TAKE · TIME · TURN · WALK · WATER · WAY · WOOD · WORK