With this spy might be quietly intrusive (4)
I believe the answer is:
nosy
'intrusive' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for nosy that I've seen before include "Inquisitive in nasal manner" , "Meddlesome" , "Intrusive in nasal way" , "Prying, snoopy" , "Showing too much curiosity about people's affairs" .)