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' is an insertion indicator.
'blue' becomes 'sad' (similar in meaning).
'raised' says the letters should be written in reverse (going up in a down clue).
'i' put into 'sad' is 'siad'.
'siad' backwards is '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" .)