Eastern trifle up on river island (4)
I believe the answer is:
eyot
'river island' is the definition.
(I've seen this before)
'eastern trifle up on' is the wordplay.
'eastern' becomes 'e'.
'trifle' becomes 'toy' (I've seen this in another clue).
'up' is a reversal indicator (in down clue: letters go upwards).
'on' says to put letters next to each other.
'toy' backwards is 'yot'.
'e'+'yot'='EYOT'
(Other definitions for eyot that I've seen before include "Little island in a river or lake" , "Small river island" , "Another term for ait" , "Fawlty Towers waiter" , "Islet found in a river, (especially in England)" .)