One in Panama, say, heading for idyllic island nation (5)
I believe the answer is:
haiti
'nation' is the definition.
(Haiti is an example)
'one in panama say heading for idyllic island' is the wordplay.
'one in' means one lot of letters goes inside another.
'panama say' becomes 'hat' (panama is a kind of hat).
'heading for' indicates taking the first letters.
'island' becomes 'i'.
The first letter of 'idyllic' is 'i'.
'hat' placed around 'i' is 'hait'.
'hait'+'i'='HAITI'
(Other definitions for haiti that I've seen before include "Country occupying part of Hispaniola" , "West Indies country associated with voodoo" , "Western part of island discovered by Columbus in 1492" , "one in the Caribbean" , "West Indies country, capital Port-au-Prince" .)