Not happy about one having to mount platform (4)
I believe the answer is:
dais
'platform' is the definition.
(dais is a kind of platform)
'not happy about one having to mount' is the wordplay.
'not happy' becomes 'sad' ('sad' is the opposite to 'happy').
'about' indicates putting letters inside.
'one having' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to mount' says the letters should be written in reverse (letters go up, as in a horse mounting).
'sad' placed around 'i' is 'siad'.
'siad' written backwards gives 'DAIS'.
Can you help me to learn more?
(Other definitions for dais that I've seen before include "stepping up here?" , "stands on this?" , "Raised floor" , "stand" , "Piece of old cloth" .)