Not in a position (to do) (6)
I believe the answer is:
unable
'not in a position to do' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for unable that I've seen before include "Lacking the necessary skills" , "Old, weak" , "Not strong enough" , "Incompetent or powerless" , "Not in a position (to)" .)