British ones are insular (5)
I believe the answer is:
isles
'british ones are insular' is the definition.
I can't tell whether this definition defines the answer.
'ones are insular' is the wordplay.
'ones' becomes 'Is' (I is the Roman numeral for one).
'are insular' becomes 'les' (I can't justify this - if you can you should believe this answer much more).
'is'+'les'='ISLES'
Can you help me to learn more?
(Other definitions for isles that I've seen before include "British locations, say?" , "Eyots" , "Hebrides, perhaps" , "eg The Hebrides" , "The Hebrides for instance" .)