Enthusiasm at the heart of Pacific nation? (4)
I believe the answer is:
zeal
'enthusiasm' is the definition.
(zeal is a kind of enthusiasm)
'the heart of pacific nation?' is the wordplay.
'the heart of' indicates the central letters.
'pacific nation?' becomes 'NewZealand' (country in the Pacific Ocean).
The middle letters of 'newzealand' are 'ZEAL'.
'at' is the link.
(Other definitions for zeal that I've seen before include "Passion" , "Fire" , "Verve" , "Fervent devotion" , "Strong feeling" .)