Serviceable (6)
'serviceable' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for usable that I've seen before include "open to exploitation" , "at one's disposal" , "Fit for purpose" , "(Of, eg, tool) employable" , "Affording some purpose" .)
(Other definitions for useful that I've seen before include "profitable" , "Handy, serviceable" , "Advantageous, serviceable" , "Handy, practical" , "Utilitarian" .)
'serviceable' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for of use that I've seen before include "Advantageous" , "Handy" , "practical" , "Helpful" , "Useful" .)