Small area of land is left reduced by 50% (4)
I believe the answer is:
isle
'small area of land' is the definition.
(I've seen this before)
'is left reduced by 50%' is the wordplay.
'reduced by 50%' suggests halving the letters.
'left' halved is 'le'.
'is'+'le'='ISLE'
(Other definitions for isle that I've seen before include "Could be Man" , "Spot (in ocean)" , "''There is an ..., a bonny ...'' (anthem of Shannon RFC)" , "See 11 Across" , "Small bit of land surrounded by water" .)