A clean heart is a heart that is pure from lusts that don’t please Allah, a heart that doesn’t believe anything that contradicts with the Quran and sunna, and doesn’t worship anyone other than Allah.
Allah the Almighty says:

“The Day whereon neither wealth nor sons will avail, Except him who brings to Allah a clean heart”.

The greatest blessing and the greatest success man can achieve is to meet Allah with a clean heart, a heart that is clean from lusts that don’t please Allah,

provided that it has a believing environment. Allah the Almighty says:

“O you who believe! Be afraid of Allah, and be with those who are true (in words and deeds) .”

[At-Taubah, 119]

Man needs to be surrounded with believers and to have a believing environment. Allah the Almighty says: 

“And keep yourself (O Muhammad PBUH) patiently with those who call on their Lord morning and afternoon, seeking His Face, and let not your eyes overlook them, desiring the pomp and glitter of the life of the world”.

If man decides to get to know Allah through the universe, and lives with the believers in good company,

he would get light and happiness from Allah so that he would see the right things as right and the false things as false. Bear in mind that happiness is priceless.