At the moment, assume one is in South American national park (9)
I believe the answer is:
snowdonia
'national park' is the definition.
(I've seen this before)
'at the moment assume one is in south american' is the wordplay.
'at the moment' becomes 'now' (I've seen this before).
'assume' becomes 'don' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'is in' is an insertion indicator.
'south american' becomes 'sa'.
'now'+'don'+'i'='nowdoni'
'nowdoni' going within 'sa' is 'SNOWDONIA'.
(Other definitions for snowdonia that I've seen before include "Britain's third national park, designated 1951" , "mountainous area" , "peaks" , "massif region" , "National park in Wales" .)