Notice the old man in study, poker-faced (7)
I believe the answer is:
deadpan
'poker-faced' is the definition.
(synonyms)
'notice the old man in study' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'the old man' becomes 'pa' (both can mean someone's father).
'in' indicates putting letters inside.
'study' becomes 'den' (den can mean an office or study).
'ad'+'pa'='adpa'
'adpa' going into 'den' is 'DEADPAN'.
(Other definitions for deadpan that I've seen before include "Poker-faced" , "Keeping a straight face" , "Betraying no emotion" , "Impassive" , "Lifeless" .)