Platform many have quit (5,4)
I believe the answer is:
stand down
'quit' is the definition.
(I've seen this before)
'platform many have' is the wordplay.
'platform' becomes 'stand' (stand is a kind of platform).
'many' becomes 'd'.
'have' becomes 'own' (to have something is to own it).
'stand'+'d'+'own'='STAND DOWN'
(Other definitions for stand down that I've seen before include "Resign (from office)" , "Withdraw; go off duty" , "Exhibition equipment packed up" , "Withdraw voluntarily, from job or competition" , "Quit" .)