A leaning cornflower may need support, but first look at how it was grown. Closely packed plants compete for light, and a pot that looked attractively full when young can become difficult to manage later.
Check the expected height of the variety. A taller cornflower is not failing merely because it is taller than a dwarf plant sold beside it.
Separate support from diagnosis
If stems are already leaning into a path, provide suitable support without tightly bunching the whole plant. Then review spacing, light and the recent feeding routine.
Look at new stems rather than expecting a bent old one to straighten completely. If each new stem is also long and weak, the growing conditions deserve attention.
Water according to the compost, not as a way to hold a plant upright. Neither an extra drink nor a strong feed corrects a crowded position. For the next sowing, allow room for the mature plants and keep the cultivar label. That makes it easier to distinguish a naturally tall variety from growth that is stretching toward light.
Related reading: Cornflower Care: Sowing, Soil, Light and Watering.