A period of time in Scottish port (3)
I believe the answer is:
ayr
'scottish port' is the definition.
(Ayr is a Scottish port town)
'a period of time' is the wordplay.
'period of time' becomes 'yr' (yr is a kind of period of time).
'a'+'yr'='AYR'
'in' is the link.
(Other definitions for ayr that I've seen before include "Scottish town, port and racecourse" , "Scottish town sounds tuneful" , "Town on Firth of Clyde" , "Scots port on Firth of Clyde" , "Scottish salmon river" .)