Handy round protective device (2,3)
I believe the answer is:
of use
'handy' is the definition.
(I've seen this before)
'round protective device' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'protective device' becomes 'fuse' (fuse is a kind of device. I am not sure about the 'protective' bit.).
'o'+'fuse'='OF USE'
Can you help me to learn more?
(Other definitions for of use that I've seen before include "Having a function" , "Serviceable" , "Useful" , "Nifty" , "Handy" .)