Ordinary circuit breaker may be helpful (2,3)
I believe the answer is:
of use
'be helpful' is the definition.
(thesaurus)
'ordinary circuit breaker' is the wordplay.
'ordinary' becomes 'o' (abbreviation e.g. O-Levels).
'circuit breaker' becomes 'fuse'.
'o'+'fuse'='OF USE'
'may' is the link.
(Other definitions for of use that I've seen before include "Handy" , "Having a function" , "Employable" , "Serviceable" , "Nifty" .)