Old electrical device, valuable (2,3)
I believe the answer is:
of use
'valuable' is the definition.
'old electrical device' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'electrical device' becomes 'fuse' (fuse is a kind of electrical device).
'o'+'fuse'='OF USE'
(Other definitions for of use that I've seen before include "Serviceable" , "practical" , "Nifty" , "Useful" , "Having a function" .)