Handy ring mains protector (2,3)
I believe the answer is:
of use
'handy' is the definition.
(I've seen this in another clue)
'ring mains protector' is the wordplay.
'ring' becomes 'O' (letter resembling a ring).
'mains protector' becomes 'fuse' (a fuse protects electrical mains).
'o'+'fuse'='OF USE'
(Other definitions for of use that I've seen before include "Helpful" , "Useful" , "Serviceable" , "Advantageous" , "Employable" .)