Notice old man 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 old man in study' is the wordplay.
'notice' becomes 'ad'.
'old man' becomes 'pa' (both can mean father).
'in' is an insertion indicator.
'study' becomes 'den' (I've seen this before).
'ad'+'pa'='adpa'
'adpa' placed into 'den' is 'DEADPAN'.
(Other definitions for deadpan that I've seen before include "showing no emotion" , "Serious-looking" , "Showing no expression" , "Blank" , "Impassive" .)