Piece of land is approaching half a mile (4)
I believe the answer is:
isle
'piece of land' is the definition.
(I've seen this before)
'is approaching half a mile' is the wordplay.
'approaching' means one lot of letters go next to another.
'half a' means to take half the letters.
'mile' with half the letters taken is 'le'.
'is'+'le'='ISLE'
(Other definitions for isle that I've seen before include "what Donne implied he wasn't" , "Of Man or Youth, for example" , "Man, for one" , "See 11 Across" , "Fair one is in shipping bulletin" .)