Platform united in wild handclap (6,3)
I believe the answer is:
launch pad
'platform' is the definition.
(launch pad is a kind of platform)
'united in wild handclap' is the wordplay.
'united' becomes 'u'.
'in' means one lot of letters goes inside another.
'wild' is an anagram indicator.
'handclap' with letters rearranged gives 'lanchpad'.
'u' inserted inside 'lanchpad' is 'LAUNCH PAD'.
(Other definitions for launch pad that I've seen before include "that's a good start" , "explorer's starting place" , "Structure on which a rocket sits" , "Rocket's starting point" , "Good place to begin" .)