Endlessly played up in small island (4)
I believe the answer is:
eyot
'small island' is the definition.
(I've seen this in another clue)
'endlessly played up' is the wordplay.
'endlessly' means to remove the last letter.
'played' becomes 'toyed' ('toy' can be a synonym of 'play').
'up' says the letters should be written backwards (in down clue: letters go upwards).
'toyed' with its last letter taken off is 'toye'.
'toye' back-to-front is 'EYOT'.
'in' acts as a link.
(Other definitions for eyot that I've seen before include "isle" , "Another term for ait" , "There's one at Chiswick" , "Islet found in a river, (especially in England)" , "land in the river" .)