This is a container clue - one word or set of letters goes inside (or around) another word or set of letters.
Find a word meaning 'left' that can go around or contain the letters that represent 'X'. The word for 'left' acts as a container, with 'X' (think Roman numerals) going inside it.
Answer: PORTENT | PORT (left, as in port side of a ship) goes around TEN (X in Roman numerals) to give PORTENT (a sign or omen).