Notice the old man in study, expressionless (7)
I believe the answer is:
deadpan
'expressionless' is the definition.
('deadpan' can be a synonym of 'expressionless')
'notice the old man in study' is the wordplay.
'notice' becomes 'ad'.
'the old man' becomes 'pa' ('pa' can be a synonym of 'old man').
'in' means one lot of letters goes inside another.
'study' becomes 'den' (I've seen this before).
'ad'+'pa'='adpa'
'adpa' going into 'den' is 'DEADPAN'.
(Other definitions for deadpan that I've seen before include "Betraying no emotion" , "Keeping a straight face" , "Showing no expression" , "Poker-faced" , "Serious-looking" .)