Returning every so often, rushes onto the island (6)
I believe the answer is:
honshu
'island' is the definition.
(Honshu is an example)
'returning every so often rushes onto the' is the wordplay.
'returning' says the letters should be written in reverse.
'every so often' means one should take alternating letters.
'rushes'+'onto'+'the'='rushesontothe'
The alternate letters of 'rushesontothe' are 'uhsnoh'.
'uhsnoh' reversed gives 'HONSHU'.
(Other definitions for honshu that I've seen before include "Main Japanese island" , "Largest of the islands forming Japan" , "The largest island unit of Japan" , "Largest island of mainland Japan" , "part of Japan" .)