Number is small and divisible by two (5)
I believe the answer is:
seven
'number' is the definition.
(seven is a kind of whole number)
'small and divisible by two' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' says to put letters next to each other.
'divisible by two' becomes 'even' (I've seen this before).
's'+'even'='SEVEN'
'is' is the link.
(Other definitions for seven that I've seen before include "Cardinal" , "Little friends of ..." , "Cube root of 343" , "Wonders of the World - Deadly Sins" , "VII" .)