Number three eight nine initially (3)
I believe the answer is:
ten
'number' is the definition.
(ten is a kind of number)
'three eight nine initially' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'three eight nine' is 'TEN'.
(Other definitions for ten that I've seen before include "Roman numeral X" , "honour" , "Base of decimal system" , "Five times two" , "PM's address, Downing St" .)