I appreciate that popular number one: it’s in South Pacific (6)
I believe the answer is:
tahiti
'in south pacific' is the definition.
(I know that Tahiti is in South Pacific)
'i appreciate that popular number one' is the wordplay.
'i appreciate that' becomes 'ta' (informal way of saying thank you).
'popular number' becomes 'hit' (hit song - number can mean a song).
'one' becomes 'i' (Roman numeral).
'ta'+'hit'+'i'='TAHITI'
'it's' acts as a link.
(Other definitions for tahiti that I've seen before include "Windward island" , "Romantic Polynesian island, capital Papeete" , "South Seas island, capital Papeete" , "Pacific island, the largest in French Polynesia" , "South Pacific island where both Robert Louis Stevenson and Paul Gauguin spent time" .)