The crossword solution for the term REIN, ESSENTIALLY is STRAP. A STRAP is a long, thin piece of material used to bind, fasten, or secure objects together. Straps are commonly used in a variety of contexts, such as to tie down luggage, secure items to a vehicle, or to hold up clothing. In the context of horse riding, a rein is a STRAP attached to the bridle of a horse and used by the rider to control the horse's movements. Thus, the solution to the crossword clue REIN, ESSENTIALLY is STRAP.