Inflexible but viable perhaps including two thousand plus order initially (9)
I believe the answer is:
immovable
'inflexible' is the definition.
(thesaurus)
'viable perhaps including two thousand plus order initially' is the wordplay.
'perhaps' indicates an anagram.
'including' is an insertion indicator.
'two thousand' becomes 'mm'.
'plus' means one lot of letters go next to another.
'initially' says to take the initial letters.
The first letter of 'order' is 'o'.
'viable' with letters rearranged gives 'ivable'.
'mm'+'o'='mmo'
'ivable' enclosing 'mmo' is 'IMMOVABLE'.
'but' acts as a link.
(Other definitions for immovable that I've seen before include "fast" , "Steadfast" , "Unyielding, fixed" , "Impassive" , "Not able to be shifted" .)