Port in endless disorder (3)
I believe the answer is:
rio
'port' is the definition.
(Rio de Janeiro is a port)
'endless disorder' is the wordplay.
'endless' means to remove the last letter.
'disorder' becomes 'riot' (riot is a kind of disorder).
'riot' with its final letter removed is 'RIO'.
'in' acts as a link.
(Other definitions for rio that I've seen before include "Manchester United player in South American city, in short?" , "Spanish word for river" , "Flying Down to -, Astaire, Rogers film" , "- Ferdinand, footballer" , "A Mexican river - like Grande" .)