"One's primary instrument" refers to the main musical instrument that a person plays or specializes in. It is the instrument they have developed the most skill and expertise in. Full definition
The Advanced Baseline Imager, or ABI, is GOES - R's primary instrument for scanning Earth's weather, oceans, and environment... (wattsupwiththat.com)
Then Israel will be restored, according to Darby, as God's primary instrument in history, the wicked will be destroyed in the final battle of Armageddon, and Christ will begin a 1,000 - year reign on earth. (religion-online.org)
Whether that means dancing the night away to Rake «n» Scrape (a traditional form of music whose primary instrument is a saw) or participating in Junkanoo (a yearly island - wide festival), we want in on the action! (kitchener-waterloo.momstown.ca)