One in blue raised platform (4)
I believe the answer is:
dais
'platform' is the definition.
(I know that dais is a type of platform)
'one in blue raised' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'blue' becomes 'sad' (similar in meaning).
'raised' says the letters should be written backwards (going up in a down clue).
'i' inserted inside 'sad' is 'siad'.
'siad' written backwards gives 'DAIS'.
(Other definitions for dais that I've seen before include "stands on this?" , "Speaker's raised platform" , "device to give some an uplift at conference?" , "stepping up here?" , "support for speaker" .)