Platform One: miserable having to go back across it (4)
I believe the answer is:
dais
'platform' is the definition.
(dais is a kind of platform)
'one miserable having to go back across it' is the wordplay.
'one' becomes 'I' (Roman numeral).
'miserable' becomes 'sad' (similar in meaning).
'having to go back' says the letters should be written in reverse.
'across it' indicates putting letters inside (some letters go across or around others).
'sad' back-to-front is 'das'.
'i' inserted within 'das' is 'DAIS'.
(Other definitions for dais that I've seen before include "stepping up here?" , "Speaker's raised platform" , "stands on this?" , "Raised floor" , "Low platform at end of hall" .)