What is Crack ?
Crack is (noun) 1. a sharp sound the crack of a whip The crack of a twig behind her made her turn round. 2. a long thin break in something hard A crack appeared in the ceiling. Her ring fell down a crack in the floorboards. The field is so dry it is full of cracks. 3. to have a crack at something to try to do something (informal.) I’ve never tried windsurfing before but I’m willing to have a crack at it. 4. a joke She made a nasty crack about his bald patch. 5. a highly addictive form of the drug cocaine 6. at (the) crack of dawn as soon as it starts to be light If we want to miss the traffic we must set off at (the) crack of dawn.(verb) 1. to make a sharp sound A twig cracked as he stepped on it. 2. to make a long thin break in something The stone cracked the glass. 3. to crack jokes to tell jokes He spent the entire lunch break cracking jokes. 4. to find out how a code works They spent months trying to crack the enemy codes.
source: Easier English, Student Dictionary Upper Intermediate Level