A port cased by five United States divers (7)
I believe the answer is:
various
'divers' is the definition.
(I've seen this before)
'a port cased by five united states' is the wordplay.
'port' becomes 'rio' (Rio de Janeiro is a port).
'cased by' indicates putting letters inside.
'five' becomes 'V' (Roman numeral).
'united states' becomes 'US' (abbreviation for the United States).
'a'+'rio'='ario'
'v'+'us'='vus'
'ario' going within 'vus' is 'VARIOUS'.
(Other definitions for various that I've seen before include "Of several different kinds" , "Manifold" , "Saviour of different types" , "More than one" , "Of different kinds or sorts" .)