Number, small, not odd (5)
I believe the answer is:
seven
'number' is the definition.
(seven is a kind of whole number)
'small not odd' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'not odd' becomes 'even' ('even' is the opposite to 'odd').
's'+'even'='SEVEN'
(Other definitions for seven that I've seen before include "Number of colours of the rainbow" , "Prime number" , "Little friends of ..." , "number of swans aswimming" , "Figure of dwarfs?" .)