First in hierarchy, number is top (6)
I believe the answer is:
height
'top' is the definition.
('height' can be a synonym of 'top')
'first in hierarchy number' is the wordplay.
'first in' suggests taking the first letters.
'number' becomes 'eight' (eight is a kind of number).
The initial letter of 'hierarchy' is 'h'.
'h'+'eight'='HEIGHT'
'is' acts as a link.
(Other definitions for height that I've seen before include "part of a patient's statistics?" , "Peak" , "Vertical extension" , "Altitude or elevation" , "Distance above ground or sea" .)