A cloverleaf portion is a type of road junction that consists of three or four curved ramps that allow vehicles to travel between two roads. It is also known as a 'trumpet interchange' due to its shape resembling a musical instrument. The term 'cloverleaf portion' is derived from the fact that the ramps form a shape that resembles a four-leaf clover. The LOOP in the crossword solution refers to the curved ramps that make up the cloverleaf portion.