Function, purpose (3)
I believe the answer is:
use
I believe this clue is a double definition.
'function' is the first definition.
(something's use is its function)
'purpose' is the second definition.
(I know that purpose can be written as use)
(Other definitions for use that I've seen before include "Employment; utilise" , "Put to a particular purpose" , "Sue for employment" , "Employ (something)" , "Expend in amusement" .)