Workroom is out for working - decorator's beginning inside (6)
I believe the answer is:
studio
'workroom' is the definition.
(I've seen this in another clue)
'is out for working decorator's beginning inside' is the wordplay.
'for working' indicates an anagram.
'beginning' says to take the initial letters.
'inside' indicates putting letters inside.
The first letter of 'decorators' is 'd'.
'is'+'out'='isout'
'isout' anagrammed gives 'stuio'.
'stuio' enclosing 'd' is 'STUDIO'.
(Other definitions for studio that I've seen before include "Workroom of an artist or photographer" , "Workplace for the practice of art" , "Here one may record" , "MGM, for example" , "Workroom for 27 down" .)