Farewell from guerrilla leader to north of European port (7)
I believe the answer is:
cheerio
'farewell' is the definition.
(cheerio is a kind of farewell)
'guerrilla leader to north of european port' is the wordplay.
'guerrilla leader' becomes 'che' (I've seen this in other clues).
'to north of' means one lot of letters go next to another (in a down clue, some letters go above others).
'european' becomes 'e' (abbreviation e.g. EU).
'port' becomes 'rio' (I've seen this before).
'che'+'e'+'rio'='CHEERIO'
'from' is the link.
(Other definitions for cheerio that I've seen before include "Say this before drinking" , "Final message" , "Ta-ta" , "Bye-bye" , "British romance imprint" .)