Set a date to enter port (5)
I believe the answer is:
radio
'set' is the definition.
(radio is a kind of set)
'a date to enter port' is the wordplay.
'a date' becomes 'ad' (AD is an example).
'to enter' is an insertion indicator.
'port' becomes 'rio' (I've seen this before).
'ad' going into 'rio' is 'RADIO'.
(Other definitions for radio that I've seen before include "Electronic medium of communication" , "Receiving apparatus" , "Broadcaster" , "broadcasting station" , "Communications medium" .)