The term 'REASON FOR A DO-OVER?' can be answered with the three letter word LET. This word is used to indicate that something should be allowed or permitted. It is often used in the context of a do-over, which is when a situation is repeated or restarted due to an error or mistake. Therefore, the answer LET can be interpreted as a reason for a do-over, as it implies that the mistake should be allowed and the situation should be restarted.