The painlessness of drivers accepting leasing arrangement (9)
I believe the answer is:
analgesia
'the painlessness' is the definition.
(I've seen this before)
'drivers accepting leasing arrangement' is the wordplay.
'drivers' becomes 'aa' (I've seen this in other clues).
'accepting' is an insertion indicator.
'arrangement' indicates anagramming the letters.
'leasing' anagrammed gives 'nalgesi'.
'aa' enclosing 'nalgesi' is 'ANALGESIA'.
'of' is the link.
(Other definitions for analgesia that I've seen before include "result of medication?" , "painless" , "The relief of pain by drugs" , "Loss of pain" , "Inability to feel pain" .)