Lair copied article filling blank (7)
I believe the answer is:
deadpan
'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 "With expressionless face" , "without a flicker" , "with emotionless face" , "Not giving anything away" , "Keeping a straight face" .)