Extremely good sailor returns to visit island (6)
I believe the answer is:
tobago
'island' is the definition.
(Caribbean island)
'extremely good sailor returns to visit' is the wordplay.
'extremely' becomes 'too' (I've seen this in other clues).
'good' becomes 'g' (abbreviation).
'sailor' becomes 'ab' (abbreviation for able seaman).
'returns' says the letters should be written backwards.
'to visit' is an insertion indicator.
'g'+'ab'='gab'
'gab' backwards is 'bag'.
'too' going around 'bag' is 'TOBAGO'.
(Other definitions for tobago that I've seen before include "Caribbean island" , "island location" , "West Indian island" , "Island near Trinidad" , "Trinidad companion island" .)