Guerrilla leader overwhelming European port for so long (7)
I believe the answer is:
cheerio
'for so long' is the definition.
(both can mean goodbye)
'guerrilla leader overwhelming european port' is the wordplay.
'guerrilla leader' becomes 'che' (guerrilla revolutionary Che Guevara).
'overwhelming' says to put letters next to each other (in a down clue, some letters go over others).
'european' becomes 'e' (abbreviation e.g. EU).
'port' becomes 'rio' (Rio de Janeiro is a port).
'che'+'e'+'rio'='CHEERIO'
(Other definitions for cheerio that I've seen before include "Informal goodbye, sounds blithe" , "Informal farewell" , "for so long!" , "A hearty farewell, begins with applause" , "Bye-bye" .)