WordNet sense 1 (someone who helps another person commit a crime):

accessary

WordNet sense 2 (clothing that is worn or carried, but not part of your main clothing):

accouterment, accoutrement

WordNet sense 3 (a supplementary component that improves capability):

add-on, appurtenance, supplement

WordNet sense 4 (furnishing added support):

adjunct, adjuvant, ancillary, appurtenant, auxiliary, subsidiary