Terribly rude about medium object (5)
I believe the answer is:
demur
'object' is the definition.
(to demur is to object to something)
'terribly rude about medium' is the wordplay.
'terribly' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'medium' becomes 'm' (as in UK clothing sizes).
'rude' with letters rearranged gives 'deur'.
'deur' enclosing 'm' is 'DEMUR'.
(Other definitions for demur that I've seen before include "Raise mild objection" , "Dissent, object" , "Waver" , "Object gently" , "Cavil, dispute" .)