Requisition (10)
'requisition' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for commandeer that I've seen before include "requisition" , "Confiscate" , "Seize" , "Appropriate" , "Take control of for military purposes" .)
(Other definitions for obtain that I've seen before include "Get one's hands on" , "Acquire or procure" , "'Get, acquire (6)'" , "Get or lay hold of" , "Come into possession of" .)