Needed as substitutes? Come off it! (5)
I believe the answer is:
bench
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for bench that I've seen before include "Exhibit at dog show" , "Pew or settle" , "Long seat for judges" , "Plain, stout work-table" , "Magistrates collectively" .)