One applauding directors — one of those on film set? (12)
I believe the answer is:
clapperboard
'one of those on film set?' is the definition.
(a clapperboard is used at the start of a take)
'one applauding directors' is the wordplay.
'one applauding' becomes 'clapper' (someone clapping their hands).
'directors' becomes 'board' (board of directors).
'clapper'+'board'='CLAPPERBOARD'
(Other definitions for clapperboard that I've seen before include "one way to call the shots?" , "Agent of filmmaker" , "part in film-making" , "Film maker's aid" , "It's used at start of filming" .)