Ales gain a sort of insensibility (9)
I believe the answer is:
analgesia
'insensibility' is the definition.
(I've seen this before)
'ales gain a sort' is the wordplay.
'sort' indicates an anagram.
'ales'+'gain'+'a'='alesgaina'
'alesgaina' anagrammed gives 'ANALGESIA'.
'of' acts as a link.
(Other definitions for analgesia that I've seen before include "Inability to feel pain" , "The relief of pain by drugs" , "result of medication?" , "Loss of pain" , "painless" .)