Caught boat regularly to circle posh island (4)
I believe the answer is:
cuba
'island' is the definition.
(Cuba is an example)
'caught boat regularly to circle posh' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'to circle' means one lot of letters goes inside another.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
The alternate letters of 'boat' are 'ba'.
'c'+'ba'='cba'
'cba' going around 'u' is 'CUBA'.
(Other definitions for cuba that I've seen before include "Castro's country" , "Caribbean nation" , "Large island discovered by Columbus, 1492" , "Hemingway's stamping-ground" , "Country famous for its cigars" .)