Strap replacing steel bat (4,4)
I believe the answer is:
seat belt
'strap' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'replacing steel bat' is the wordplay.
'replacing' indicates anagramming the letters (placing again).
'steel'+'bat'='steelbat'
'steelbat' anagrammed gives 'SEAT-BELT'.
Can you help me to learn more?
(Other definitions for seat belt that I've seen before include "It must be worn in car" , "Strapping thing" , "Car safety feature" , "Protective strap in car" , "restraining device" .)