Notice dad in study, poker-faced (7)
I believe the answer is:
deadpan
'poker-faced' is the definition.
('deadpan' can be a synonym of 'poker-faced')
'notice dad in study' is the wordplay.
'notice' becomes 'ad'.
'dad' becomes 'pa' ('pa' can be a synonym of 'dad').
'in' means one lot of letters goes inside another.
'study' becomes 'den' (I've seen this before).
'ad'+'pa'='adpa'
'adpa' put into 'den' is 'DEADPAN'.
(Other definitions for deadpan that I've seen before include "Serious-looking" , "Lifeless" , "without a flicker" , "Betraying no emotion" , "Not giving anything away" .)