Diligence essential to NZ's expansion (4)
I believe the answer is:
Here is my best explanation:
'diligence' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'essential to nz's expansion' is the wordplay.
'essential to' says to take the centre.
'expansion' becomes 'eal' (I am not sure about this - if you are sure you should believe this answer much more).
The middle of 'nzs' is 'z'.
(Other definitions for zeal that I've seen before include "Extreme enthusiasm", "Ardour", "Eagerness and devotion", "Fervour", "fire", "Fanaticism", "Fervent devotion", "6 [ENTHUSIASM]", "Verve" and "Militancy".)