Guard local Post Office from parasites (6)
I believe the answer is:
police
'guard local' is the definition.
'police' can be an answer for 'guard' (policing is a kind of guarding). I'm not sure about the 'local' bit.
'post office from parasites' is the wordplay.
'post office' becomes 'po' (abbreviation).
'from' means one lot of letters go next to another.
'parasites' becomes 'lice' (lice are parasitic insects).
'po'+'lice'='POLICE'
Can you help me to learn more?
(Other definitions for police that I've seen before include "Bobby" , "Branch of the security forces" , "... custodians of law and order" , "Emergency service" , "Boys in blue" .)