Fit for purpose (6)
'fit for purpose' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for usable that I've seen before include "(Of, eg, tool) employable" , "exploited" , "Functional, handy" , "open to exploitation" , "not to be discarded?" .)
(Other definitions for proper that I've seen before include "Actual" , "Seemly" , "Appropiate, right" , "Appropriate according to rules" , "well-behaved" .)