Forgetfulness is a mean condition (7)
I believe the answer is:
amnesia
'forgetfulness' is the definition.
(I've seen this before)
'is a mean condition' is the wordplay.
'condition' indicates an anagram (I've seen 'conditions' mean this (letters must be in a new condition)).
'is'+'a'+'mean'='isamean'
'isamean' with letters rearranged gives 'AMNESIA'.
(Other definitions for amnesia that I've seen before include "Clinical loss of memory" , "memory problem" , "Defect in memory" , "Main sea (anag.)" , "As a mine (anag)" .)