Notice father in study, expressionless (7)
I believe the answer is:
deadpan
'expressionless' is the definition.
(I know that expressionless can be written as deadpan)
'notice father in study' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'father' becomes 'pa' (pa can mean dad or father).
'in' indicates putting letters inside.
'study' becomes 'den' (term for a study or office).
'ad'+'pa'='adpa'
'adpa' put inside 'den' is 'DEADPAN'.
(Other definitions for deadpan that I've seen before include "Betraying no emotion" , "Keeping a straight face" , "with emotionless face" , "Lifeless" , "without a flicker" .)