Have on grand frock (4)
I believe the answer is:
gown
'frock' is the definition.
(gown is a kind of frock)
'have on grand' is the wordplay.
'have' becomes 'own' ('own' can be a synonym of 'have').
'on' says to put letters next to each other.
'grand' becomes 'g' ('G' can be a synonym of 'grand'**).
'own' put after 'g' is 'GOWN'.
(Other definitions for gown that I've seen before include "Formal frock, hospital garb and ceremonial dress" , "Academic robe" , "An evening dress" , "Dress for formal occasions" , "Ball dress" .)