The answer will depend on a few details of the car, including it's age.
Cars first used on or after 1st April 1986 MUST be fitted with one side repeater indicator on each side. Instead of a separate lamp, the side repeater might be part of the front direction indicator if it includes a wraparound lens.
Based on the assumption that the original wings include separate repeaters, is the car in question post 1986 and not equipped with wraparound lenses? If so then the missing repeater will be an MOT failure.