Forgetting a name is unusual (7)
I believe the answer is:
amnesia
'forgetting' is the definition.
(medical condition affecting the memory)
'a name is unusual' is the wordplay.
'unusual' indicates an anagram.
'name'+'is'='nameis'
'nameis' anagrammed gives 'mnesia'.
'a'+'mnesia'='AMNESIA'
(Other definitions for amnesia that I've seen before include "illness" , "Partial or total loss of memory" , "memory problem" , "Completely forgetting" , "Medical condition" .)