Small river island (4)
I believe the answer is:
'small river island' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for eyot that I've seen before include "Another term for ait" , "Islet found in a river, (especially in England)" , "Little island in a river or lake" , "Small river island" , "There's one at Chiswick" .)