Thief who enters a building to steal (7)
I believe the answer is:
burglar
'thief who enters a building to steal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for burglar that I've seen before include "Intruder with intent to steal" , "A stealer from homes" , "House robber" , "One breaking in" , "Building robber" .)