Second matching number (5)
I believe the answer is:
seven
'second matching number' is the definition.
'seven' can be an answer for 'number' (seven is a kind of whole number). I'm not sure about the 'second matching' bit.
'matching number' is the wordplay.
'matching' becomes 'seve' (I can't explain this - if you can you should give a lot more credence to this answer).
'number' becomes 'n' (n can represent any number).
'seve'+'n'='SEVEN'
Can you help me to learn more?
(Other definitions for seven that I've seen before include "Prime number" , "Number of sisters in Sussex" , "Number of Wonders of the World" , "Numeral" , "Number of sins, seas or dwarves" .)