Cycle endlessly in one atoll (6)
I believe the answer is:
bikini
'atoll' is the definition.
(I've seen this before)
'cycle endlessly in one' is the wordplay.
'cycle' becomes 'bike' (synonyms).
'endlessly' means to remove the last letter.
'one' becomes 'I' (Roman numeral).
'bike' with its last letter taken off is 'bik'.
'bik'+'in'+'i'='BIKINI'
(Other definitions for bikini that I've seen before include "Pacific atoll, used for US nuclear weapons tests" , "gear in two pieces" , "Kind of swimsuit" , "Dress" , "Small two-piece bathing costume" .)