Inability to coordinate drivers locking vehicle (6)
I believe the answer is:
ataxia
'inability to coordinate' is the definition.
'drivers locking vehicle' is the wordplay.
'drivers' becomes 'aa' (I've seen this in other clues).
'locking' is an insertion indicator.
'vehicle' becomes 'taxi' (taxi is a kind of vehicle**).
'aa' placed around 'taxi' is 'ATAXIA'.
(Other definitions for ataxia that I've seen before include "Inability to coordinate actions" , "motor problem" , "Inability to coordinate voluntary muscle movements" , "Inability to manoeuvre" .)