The crossword solution for the term 'ESTER OPENING' is 'POLY'. An ester opening is a type of chemical reaction in which an ester is broken down into its component parts. This reaction is catalyzed by an acid or base, and the resulting products are an alcohol and a carboxylic acid. The prefix 'poly' means many, so the term 'polyester opening' refers to a reaction in which multiple esters are broken down into their component parts.