Diligence essential to NZ's expansion (4)
I believe the answer is:
zeal
'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'.
'z'+'eal'='ZEAL'
(Other definitions for zeal that I've seen before include "6 [ENTHUSIASM]" , "Zing" , "Strong feeling" , "Interest that's definitely not diminished" , "Zest, ardour" .)