It possibly includes a success in the S. Pacific (6)
I believe the answer is:
tahiti
'the s pacific' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'it possibly includes a success' is the wordplay.
'possibly' is an anagram indicator.
'includes' indicates putting letters inside.
'success' becomes 'hit' (a hit is a commercial success).
'it' anagrammed gives 'ti'.
'a'+'hit'='ahit'
'ti' going around 'ahit' is 'TAHITI'.
'in' is the link.
Can you help me to learn more?
(Other definitions for tahiti that I've seen before include "Society member" , "Gauguin's Pacific isle" , "South Pacific island, capital Papeete" , "Polynesian island where Gauguin lived" , "Pacific island, the largest in French Polynesia" .)