Decoration in one room (5)
I believe the answer is:
award
'decoration' is the definition.
(I've seen this before)
'one room' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'room' becomes 'ward' (I've seen this before).
'a'+'ward'='AWARD'
'in' acts as a link.
(Other definitions for award that I've seen before include "Grant - prize" , "Perhaps Tony" , "Adjudge" , "Bestow" , "medal, say" .)