Is an island in the Thames! (4)
I believe the answer is:
isis
'the thames' is the definition.
'is an island' is the wordplay.
'an island' becomes 'is'.
'is'+'is'='ISIS'
'in' is the link.
(Other definitions for isis that I've seen before include "Goddess of the rainbow" , "Oxford river; goddess" , "Oxfordshire river" , "Egyptian fertility goddess, mother of Horus" , "Revered figure" .)