To be clean at heart is to give happiness to others. 

Expression: The one who has a clean heart is the one who always tries to do the best for those with whom he comes in contact. Thus, the person develops the ability to accept others as they are and ignore anything wrong done by them. Instead, he is able to do the right action without losing the balance. So such a person brings happiness for himself and for others through every action he performs. 

Experience: When I have a clean heart I am able to have an experience of my inner qualities. I am able to enjoy the beauty of the different relationships, each relationship and each person being unique. Thus others are able to get in touch with their inner beauty too. So there is happiness experienced by all.