Feasible being without a room at first for changing (8)
I believe the answer is:
variable
'changing' is the definition.
(thesaurus)
'feasible being without a room at first' is the wordplay.
'feasible' becomes 'viable' ('viable' can be a synonym of 'feasible'**).
'being without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'at first' indicates taking the first letters.
The initial letter of 'room' is 'r'.
'a'+'r'='ar'
'viable' placed around 'ar' is 'VARIABLE'.
'for' acts as a link.
(Other definitions for variable that I've seen before include "Prone to change" , "Inconsistent" , "Mutable" , "Fickle" , "Liable to change" .)