Is half left in the water (4)
I believe the answer is:
isle
'in the water' is the definition.
(I've seen this before)
'is half left' is the wordplay.
'half' means to take half the letters.
'left' halved is 'le'.
'is'+'le'='ISLE'
(Other definitions for isle that I've seen before include "it's offshore" , "This is sceptred!" , "Perspire" , "Fair ---, knitting style" , "Man, maybe" .)