written agreement vs instrument

written agreement and instrument both are nouns.

Word	Noun	Adjective	Verb	Adverb
written agreement	Yes	No	No	No
instrument	Yes	No	Yes	No

As nouns, instrument is a hypernym of written agreement; that is, instrument is a word with a broader meaning than written agreement:

written agreement: a legal document summarizing the agreement between parties
instrument: (law) a document that states some contractual relationship or grants some right

Other hypernyms of written agreement include legal document, legal instrument, official document, agreement, understanding.

written agreement (noun)	instrument (noun)
a legal document summarizing the agreement between parties	the means whereby some act is accomplished
	a device that requires skill for proper use
	any of various devices or contrivances that can be used to produce musical tones or sounds
	(law) a document that states some contractual relationship or grants some right
	the semantic role of the entity (usually inanimate) that the agent uses to perform an action or start a process
	a person used by another to gain an end

written agreement (verb)	instrument (verb)
	address a legal document to
	write an instrumental score for
	equip with instruments for measuring, recording, or controlling