Roof party -- I object below (4)
I believe the answer is:
dome
'roof' is the definition.
(dome is a kind of roof)
'party i object below' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'i object' becomes 'me'.
'below' says to put letters next to each other.
'do'+'me'='DOME'
(Other definitions for dome that I've seen before include "Rounded vault of a roof" , "Hemispherical roof" , "Hemispherical structure" , "A stately building" , "millennium feature" .)