A number, its ending divisible by 2 (5)
I believe the answer is:
seven
'a number' is the definition.
(seven is a kind of whole number)
'its ending divisible by 2' is the wordplay.
'ending' indicates one should take the final letters (the ending of the word).
'divisible by 2' becomes 'even' (I've seen this in another clue).
The final letter of 'its' is 's'.
's'+'even'='SEVEN'
(Other definitions for seven that I've seen before include "See 1" , "number of swans aswimming" , "Prime number" , "Little friends of ..." , "Number of dwarfs (or deadly sins)" .)