Passport, maybe, on British island, showing particular expression (5)
I believe the answer is:
'particular expression' is the definition.
(an idiom is an expression or phrase)
'passport maybe on british island' is the wordplay.
'passport maybe' becomes 'ID' (a passport is identification).
'on' says to put letters next to each other.
'british island' becomes 'IOM' (Isle of Man).
'showing' is the link.
(Other definitions for idiom that I've seen before include "Manner of speaking or writing" , "Dialect, vernacular" , "Expression that's characteristic of particular group" , "Eg, over the moon" , "Figure of speech" .)