Stay behind on island one invades (6)
I believe the answer is:
remain
'stay behind' is the definition.
(remain can mean to stay behind)
'on island one invades' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'island' becomes 'man' (the Isle of Man is an example).
'one' becomes 'i' (Roman numeral).
'invades' indicates putting letters inside.
'man' placed around 'i' is 'main'.
're'+'main'='REMAIN'
(Other definitions for remain that I've seen before include "Stay, don't go" , "Stay (in a place)" , "Stay behind" , "Survive" , "Continue in position" .)