noun

a line on the surface of something along which it has split without breaking apart.

a hairline crack down the middle of the glass

split,fissure,crevice,break,fracture,rupture,breach,rift,cleft,slit,chink,gap,cranny,interstice,crazing

verb

break or cause to break without a complete separation of the parts.

the ice all over the bog had cracked

split,fracture,fissure,rupture,break,snap,cleave,craze

adjective

very good or skilful.

he is a crack shot

expert,skilled,skilful,masterly,virtuoso,master,consummate,proficient,accomplished,talented,gifted,adept,adroit,deft,dexterous,able,good,competent,capable,efficient,experienced,seasoned,trained,practised,professional,polished,well versed,versed,magnificent,brilliant,splendid,marvellous,impressive,excellent,formidable,outstanding,first-class,first-rate,fine,deadly,great,mean,wicked,stellar,nifty,ace,wizard,a dab hand at,crackerjack,compleat,habile,shit-hot