clutter up vs clutter

clutter up is not a noun while clutter is a noun.

Word	Noun	Adjective	Verb	Adverb
clutter up	No	No	Yes	No
clutter	Yes	No	Yes	No

As verbs, clutter up and clutter are synonyms defined as:

clutter up (noun)	clutter (noun)
	unwanted echoes that interfere with the observation of signals on a radar screen
	a confused multitude of things

clutter up (verb)	clutter (verb)
fill a space in a disorderly way	fill a space in a disorderly way