To "cling" is to hang on, to overindulge in another's self-interest. Usually the clinger forgoes his or her own destiny, all the while supplying love and fascination with the primary. Women are usually the ones doing the clinging, and usually the reason is very, very familiar...low self esteem. When you don't have a life of your own, when you agree to set aside your aspirations and desires to the whims of another, you are a prime candidate for the adjective...clingy.