If somebody is a C cup but they have no cleavage, is there any way to achieve that?

Cleavage cannot be built. With or without breast implants, clothes create the cleavage. Having a starting C cup volume will make it easier in clothes to create the look you desire.
Dr Ted Eisenberg, Board Certified Plastic Surgeon and AuthorRealSelf Distinguished Hall of Fame Inductee

Unfortunately, you have not provided photographs but a breast lift can increase cleavage.  The technique I recommend is The Bellesoma Method.  This will reshape your breast tissue creating upper pole fullness without implants, elevate them higher on the chest wall and more medial to increase your cleavage.  Vertical scars are avoided, nipple sensation and the ability to breast feed are maintained.  If you have a very wide chest and a very small C cup it may be difficult to move the breasts medial enough and achieve cleavage.  However, that is a very rate situation.Best Wishes,Gary Horndeski, M.D.