As in a leg a curious absence of pain (9)
I believe the answer is:
analgesia
'absence of pain' is the definition.
(I've seen this before)
'as in a leg a curious' is the wordplay.
'curious' indicates anagramming the letters.
'as'+'in'+'a'+'leg'+'a'='asinalega'
'asinalega' with letters rearranged gives 'ANALGESIA'.
(Other definitions for analgesia that I've seen before include "The relief of pain by drugs" , "Inability to feel pain" , "Loss of pain" , "No hard feelings" , "painless" .)