Notice the old man in study being po-faced (7)
I believe the answer is:
deadpan
'po-faced' is the definition.
Both the definition and answer are adjectives.
Maybe you can see an association between them that I don't see?
'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' (term for a study or office).
'ad'+'pa'='adpa'
'adpa' inserted into 'den' is 'DEADPAN'.
'being' is the link.
Can you help me to learn more?
(Other definitions for deadpan that I've seen before include "Serious-looking" , "Poker-faced" , "Blank" , "Keeping a straight face" , "Dry" .)