Penetrating piece (7)
I believe the answer is:
cutting
I believe this clue is a double definition.
'penetrating' is the first definition.
(both can mean very hurtful or having a big impact)
'piece' is the second definition.
(a piece cut off from something)
(Other definitions for cutting that I've seen before include "article removed from publication" , "piece used for plant propagation" , "Sharp (remark); piece kept from paper" , "article removed from newspaper" , "Excavation for road" .)