Possess dress, not grand (3)
I believe the answer is:
own
'possess' is the definition.
('own' can be a synonym of 'possess')
'dress not grand' is the wordplay.
'dress' becomes 'gown' (I've seen this before).
'not' indicates named letters should be taken away.
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'gown' with 'g' taken out is 'OWN'.
(Other definitions for own that I've seen before include "Acknowledge" , "Allow" , "Be in legal possession of (3)" , "Private" , "Posses" .)