WordNet sense 1 (an extra component of a machine or other apparatus):

spare part

WordNet sense 2 (an extra car wheel and tire for a four-wheel vehicle):

fifth wheel

WordNet sense 3 (refrain from harming):

save

WordNet sense 4 (give up what is not strictly needed):

give up, dispense with, part with

WordNet sense 5 (not taken up by scheduled activities):

free

WordNet sense 6 (not held or filled or in use):

unoccupied

WordNet sense 7 (more than is needed, desired, or required):

excess, extra, redundant, supererogatory, superfluous, supernumerary, surplus

WordNet sense 8 (not necessary):

unneeded

WordNet sense 9 (thin and fit):

trim

WordNet sense 10 (lacking excess flesh):

lean, thin

WordNet sense 11 (lacking in amplitude or quantity):

bare, scanty

WordNet sense 12 (deficient in amount or quality or extent):

meager, meagerly, stingy, scrimpy, stingy

WordNet sense 13 (lacking embellishment or ornamentation):

plain, unembellished, unornamented

WordNet sense 14 (not decorated with something to increase its beauty or distinction):

unadorned, undecorated