Major port in south-east America (7)
I believe the answer is:
serious
'major' is the definition.
(thesaurus)
'port in south-east america' is the wordplay.
'port' becomes 'rio' (I've seen this before).
'in' is an insertion indicator.
'south east' becomes 'se'.
'america' becomes 'us' (abbreviation for the United States).
'se'+'us'='seus'
'rio' put inside 'seus' is 'SERIOUS'.
(Other definitions for serious that I've seen before include "Stern" , "Earnest - important" , "Important - solemn" , "Earnest, sincere" , "Not light or frivolous" .)