Notice father in study, without expression (7)
I believe the answer is:
deadpan
'without expression' is the definition.
'notice father in study' is the wordplay.
'notice' becomes 'ad'.
'father' becomes 'pa' (pa is a kind of father).
'in' indicates putting letters inside.
'study' becomes 'den' (I've seen this before).
'ad'+'pa'='adpa'
'adpa' inserted within 'den' is 'DEADPAN'.
(Other definitions for deadpan that I've seen before include "With expressionless face" , "Lifeless" , "Not giving anything away" , "Serious-looking" , "Betraying no emotion" .)