The crossword solution for the term 'START OF A POPULAR PALINDROME' is 'ABLE', which is a four-letter word. The word 'ABLE' is often used to describe someone or something that has the power, skill, or means to do something. It can also be used to describe someone or something that is capable of being done or accomplished. In this case, it is the start of a popular palindrome, which is a word, phrase, or sentence that reads the same forward and backward.