The term 'ONE OF A DEADLY SEPTET' can be solved by the crossword clue 'SIN' (3 letters). This phrase is a reference to the seven deadly sins, which are a set of seven vices that have been recognized since early Christian times. The seven deadly sins are pride, envy, gluttony, lust, anger, greed, and sloth. Each of these sins is considered to be a major transgression against God and humanity. Therefore, the phrase 'ONE OF A DEADLY SEPTET' refers to any one of these seven sins.