cracking vs slap-up

cracking is a noun but slap-up is not a noun.

Word	Noun	Adjective	Verb	Adverb
cracking	Yes	Yes	No	No
slap-up	No	Yes	No	No

As adjectives, cracking and slap-up are synonyms defined as:

Other synonyms of cracking include bang-up, bully, corking, dandy, great, groovy, keen, neat, nifty, not bad, old, peachy, smashing, swell.

cracking (noun)	slap-up (noun)
the act of cracking something
a sudden sharp noise
the process whereby heavy molecules of naphtha or petroleum are broken down into hydrocarbons of lower molecular weight (especially in the oil-refining process)

cracking (adjective)	slap-up (adjective)
excellent	excellent