Keep actors in front of full theatre ultimately (6)
I believe the answer is:
castle
'keep' is the definition.
(I've seen this before)
'actors in front of full theatre ultimately' is the wordplay.
'actors' becomes 'cast' (group of actors).
'in front of' is a charade indicator (letters next to each other) (some letters go in front of others).
'ultimately' says to take the final letters.
The final letters of 'full theatre' are 'le'.
'cast'+'le'='CASTLE'
(Other definitions for castle that I've seen before include "Imposing residence, perhaps fortified" , "E.g. Harlech, Beaumaris, Windsor" , "Fort" , "Chess piece and fortified building" , "Move king to safety - stronghold" .)