Political party's infiltrated by 'double 0' spy (5)
I believe the answer is:
snoop
'spy' is the definition.
(I know that snoop is a type of spy)
'political party's infiltrated by double 0' is the wordplay.
'political party's' becomes 'snp' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'infiltrated by' is an insertion indicator.
'double' shows that letters should be duplicated.
'0' becomes 'o' (I've seen this in other clues).
'o' duplicated is 'oo'.
'snp' enclosing 'oo' is 'SNOOP'.
Can you help me to learn more?
(Other definitions for snoop that I've seen before include "-- Dogg, US rapper; spy" , "Investigate; pry" , "Stick one's nose in" , "Furtively look around" , "Be nosy" .)