One auction disposing of a piece of land (4)
I believe the answer is:
isle
'piece of land' is the definition.
(I've seen this before)
'one auction disposing of a' is the wordplay.
'one' becomes 'i' (Roman numeral).
'auction' becomes 'sale' (type of sale).
'disposing of' is a deletion indicator.
'sale' with 'a' removed is 'sle'.
'i'+'sle'='ISLE'
(Other definitions for isle that I've seen before include "'This scepter'd - - (Shakespeare)" , "Lilliput, say" , "Eg, Sheppey" , "Dogs, maybe" , "one has water all round" .)