Float in Irish port (4)
I believe the answer is:
cork
This clue is a double definition.
'float' is the first definition.
(cork is a kind of float)
'irish port' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for cork that I've seen before include "Bung for a bottle" , "Tree bark from the South of Ireland?" , "Buoyant bark" , "stop" , "Stopper; Irish city" .)