Nationals from banks of Nile roam south, full of enthusiasm (3,10)

'nationals' is the definition.
(nationals or citizens of New Zealand)

'banks of nile roam south full of enthusiasm' is the wordplay.
'banks of' suggests removing the centre (the banks are the sides of the word).
'roam' becomes 'wander' (synonyms).
'south' becomes 'S' (synonyms).
'full of' is an insertion indicator.
'enthusiasm' becomes 'zeal' (zeal is a kind of enthusiasm).
'nile' with its middle taken out is 'ne'.
'wander'+'s'='wanders'
'wanders' placed around 'zeal' is 'wzealanders'.
'ne'+'wzealanders'='NEW ZEALANDERS'

'from' acts as a link.