Platform is said to be unstable (4)
I believe the answer is:
dais
'platform' is the definition.
(I know that dais is a type of platform)
'said to be unstable' is the wordplay.
'to be unstable' indicates anagramming the letters.
'said' is an anagram of 'DAIS'.
'is' is the link.
(Other definitions for dais that I've seen before include "Raised floor" , "device to give some an uplift at conference?" , "support for speaker" , "Said to be a low platform" , "stage" .)