The setter's going around South American bottleneck (7)
I believe the answer is:
isthmus
'bottleneck' is the definition.
(thesaurus)
'the setter's going around south american' is the wordplay.
'the setter's' becomes 'im' (referring to the person who set the crossword).
'going around' is an insertion indicator.
'south' becomes 'sth' (abbreviation for south).
'american' becomes 'us' (abbreviation for United States).
'im' enclosing 'sth' is 'isthm'.
'isthm'+'us'='ISTHMUS'
(Other definitions for isthmus that I've seen before include "Such a narrow area" , "Narrow neck of land joining two larger portions" , "Neck of land connecting larger areas" , "Narrow linking piece of land" , "Narrow strip of land with water on two sides" .)