spying vs undercover work

spying and undercover work both are nouns.

Word	Noun	Adjective	Verb	Adverb
spying	Yes	No	No	No
undercover work	Yes	No	No	No

As nouns, spying and undercover work are synonyms defined as:

spying and undercover work: the act of keeping a secret watch for intelligence purposes

spying (noun)	undercover work (noun)
the act of keeping a secret watch for intelligence purposes	the act of keeping a secret watch for intelligence purposes
keeping a secret or furtive watch
the act of detecting something; catching sight of something