Old safety mechanism is valuable (2,3)
I believe the answer is:
of use
'is valuable' is the definition.
(I've seen this in another clue)
'old safety mechanism' is the wordplay.
I cannot quite see how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found in the answer.
The remaining letters 'fuse' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for of use that I've seen before include "Having a function" , "Serviceable" , "Helpful" , "Nifty" , "Useful" .)