Extremely small, I leave Irish port (5)
I believe the answer is:
sligo
'irish port' is the definition.
(I've seen this in another clue)
'extremely small i leave' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'leave' becomes 'go' (to go is to depart or leave).
'small' with its centre taken out is 'sl'.
'sl'+'i'+'go'='SLIGO'
(Other definitions for sligo that I've seen before include "NW Irish county" , "Irish county and town" , "part of Ireland" , "Southern Irish county" , "Irish seaport and county" .)