WordNet sense 1 (a person used by another to gain an end):

cat's-paw, pawn

WordNet sense 2 ((law) a document that states some contractual relationship or grants some right):

legal document, legal instrument, official document

WordNet sense 3 (the semantic role of the entity (usually inanimate) that the agent uses to perform an action or start a process):

instrumental role

WordNet sense 4 (any of various devices or contrivances that can be used to produce musical tones or sounds):

musical instrument

WordNet sense 5 (write an instrumental score for):

instrumentate

WordNet sense 6 (the means whereby some act is accomplished):

tool