Lair copied article filling blank (7)
'blank' is the definition.
(I've seen this before)
'lair copied article filling' is the wordplay.
'lair' becomes 'den' (synonyms).
'copied article' becomes 'adpa' (I can't justify this - if you can you should believe this answer much more).
'filling' indicates putting letters inside.
'den' going around 'adpa' is 'DEADPAN'.
Can you help me to learn more?
(Other definitions for deadpan that I've seen before include "Lifeless", "with emotionless face", "Serious-looking", "Impassive", "Blank".)