Veg sent back by European port is passed out (6,4)

'is passed out' is the definition.
(to keel over is to pass out or fall down)

'veg sent back by european port' is the wordplay.
'veg' becomes 'leek' (leek is a kind of vegetable).
'sent back' says the letters should be written backwards.
'by' is a charade indicator (letters next to each other).
'european' becomes 'E' (abbreviation e.g. EU).
'port' becomes 'dover' (English port).
'leek' in reverse letter order is 'keel'.
'keel'+'e'+'dover'='KEELED OVER'