Heartlessly ransacks building sites (4)
I believe the answer is:
lots
'building sites' is the definition.
(I've seen this in another clue)
'heartlessly ransacks' is the wordplay.
'heartlessly' means to remove the middle letters.
'ransacks' becomes 'loots' (I've seen this in another clue).
'loots' with its centre removed is 'LOTS'.
(Other definitions for lots that I've seen before include "Large number or amount" , "may be drawn" , "a crowd?" , "Loads" , "Units sold at auctions - plenty of them" .)