Port one gets in casks (5)
I believe the answer is:
tunis
'port' is the definition.
(I know that Tunis is a port)
'one gets in casks' is the wordplay.
'one' becomes 'i' (Roman numeral).
'gets in' is an insertion indicator.
'casks' becomes 'tuns' (tun is a kind of cask).
'i' inserted within 'tuns' is 'TUNIS'.
(Other definitions for tunis that I've seen before include "North African location" , "North African port and capital" , "Tom, Dick or Harry" , "Capital of a republic in north Africa" , "Carthage lies near this N. African city" .)