Compiler pens almost unusual style (4)
I believe the answer is:
mode
'style' is the definition.
('mode' can be a synonym of 'style')
'compiler pens almost unusual' is the wordplay.
'compiler' becomes 'me' (the person who is writing these clues).
'pens' is an insertion indicator.
'almost unusual' becomes 'od' (I can't justify this - if you can you should believe this answer much more).
'me' going around 'od' is 'MODE'.
Can you help me to learn more?
(Other definitions for mode that I've seen before include "Way of operation" , "Trend" , "Way, method" , "Style of dome" , "Fashion; way of performing" .)