It could be spare for diners (3)
I believe the answer is:
rib
This clue could be a double definition.
'it' is the first definition.
The definition suggests a singular noun which matches the answer.
'spare for diners' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
I also know that 'spare' relates to this answer.
'could be' acts as a link.
Can you help me to learn more?
(Other definitions for rib that I've seen before include "Slender bone" , "Curved bone joined to the spine" , "Cut of meat" , "Part of vault" , "structural member" .)