An apple is not enough for a meal, what are you eating for your other meals that you are finding yourself full for lunch?
What are your stats? (age, height, weight, BMR, activity level)
I have some snacks for lunch but then again, I snack between all meals too and still have enough cals in a day for health and for a deficit. If smaller meals is your preference is just m ake sure you have more of them, I sometimes have 3 meals a day with 1-2 snacks, or 4-6 mini meals.
Would you say snacking on the fruit and veggies accounts for your 2-4servings daily fruit intake and 3-5servings daily veg intake but still by the end of the day get your dairy (2-3), protein (2-3) and carbs (6-11)? give or take? I would think you are more ideal eater than I, if you do and even if it is not completely spot on. We all can/could do with improvements in our diets I am sure :)
Or is it you worry you, cause you don't have that meal, are missing servings you should have and you are worried about it? If so try adding the servings you are missing, if you were missing dairy, could you sneak a glass of milk or some yogurt in your mid-day snacking? Short on protein adding some nuts or maybe if it is carbs a piece of toast?
Or is it meeting a calorie intake (like getting to eat your BMR)? Naturally having a hard time with eating certain foods but want to keep healthy. Like would you like to have a low carb diet but want to do it right? etc. that sort of thing.