Emperor, possessed, managed to capture island (7)
I believe the answer is:
hadrian
'emperor' is the definition.
(Roman emperor)
'possessed managed to capture island' is the wordplay.
'possessed' becomes 'had' (to have something is to possess it).
'managed' becomes 'ran' (I've seen this before).
'to capture' means one lot of letters goes inside another.
'island' becomes 'i'.
'ran' enclosing 'i' is 'rian'.
'had'+'rian'='HADRIAN'
(Other definitions for hadrian that I've seen before include "Roman emperor who built the British wall" , "His wall crosses northern Britain" , "Roman emperor who visited Britain" , "Roman emperor and wallbuilder" , "Wall-building Roman emperor" .)