Head of security near to troubled US politician (7)
I believe the answer is:
senator
'us politician' is the definition.
(the US has a senate)
'head of security near to troubled' is the wordplay.
'head of' says to take the initial letters.
'troubled' indicates an anagram.
The first letter of 'security' is 's'.
'near'+'to'='nearto'
'nearto' with letters rearranged gives 'enator'.
's'+'enator'='SENATOR'
(Other definitions for senator that I've seen before include "politician going wrong" , "Legislative body member" , "Treason committed by member of Upper House" , "Upper House member involved in treason" , "Clinton, perhaps" .)