Danger! Noun penned by this compiler genius (6)
I believe the answer is:
menace
'danger' is the definition.
(menace is a kind of danger)
'noun penned by this compiler genius' is the wordplay.
'noun' becomes 'n'.
'penned by' means one lot of letters goes inside another.
'this compiler' becomes 'me' (the person who is writing these clues).
'genius' becomes 'ace' ('ace' can be a synonym of 'genius').
'me'+'ace'='meace'
'n' placed inside 'meace' is 'MENACE'.
(Other definitions for menace that I've seen before include "Threat or severe nuisance" , "Threaten with harm or danger" , "ominous atmosphere" , "Intimidation" , "peril" .)