Number that's square and a multiple of 2 (5)
I believe the answer is:
seven
'number' is the definition.
(seven is a kind of whole number)
'square and a multiple of 2' is the wordplay.
'square' becomes 'even' (both mean 'owing nothing' as in 'we're all square').
'and' means one lot of letters go next to another.
'a multiple of 2' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'even' put after 's' is 'SEVEN'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for seven that I've seen before include "Number (of stars in the sky?)" , "Little friends of ..." , "Figure of dwarfs?" , "Dwarfs or Wonders of the World?" , "Cardinal" .)