Farewell from guerrilla leader going over European port (7)
I believe the answer is:
cheerio
'farewell' is the definition.
(I know that cheerio is a type of farewell)
'guerrilla leader going over european port' is the wordplay.
'guerrilla leader' becomes 'che' (guerrilla revolutionary Che Guevara).
'going over' says to put letters next to each other.
'european' becomes 'E' (abbreviation e.g. EU).
'port' becomes 'rio' (Rio de Janeiro is a port).
'che'+'e'+'rio'='CHEERIO'
'from' is the link.
(Other definitions for cheerio that I've seen before include "British romance imprint" , "good health" , "Expression used when leaving" , "Say this before drinking" , "Ta-ta" .)