Start to penetrate (5)
I believe the answer is:
enter
I believe this is a double definition.
'start' is the first definition.
(I know that enter is a more specific form of take office)
'penetrate' is the second definition.
(I know that penetrate is a more specific form of enter)
'to' acts as a link.
(Other definitions for enter that I've seen before include "Introduce" , "put in for a race" , "Set foot in" , "Gain access to" , "Come or go in" .)