clutter vs clutter up

clutter is a noun but clutter up is not a noun.

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

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

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

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