Warmonger's kin, not oddly, with a failure to remember the past (7)
I believe the answer is:
amnesia
'failure to remember the past' is the definition.
'warmonger's kin not oddly with a' is the wordplay.
'not oddly' means one should take alternating letters (odd letters are removed).
'with' says to put letters next to each other.
'warmongers'+'kin'='warmongerskin'
The alternating letters of 'warmongerskin' are 'amnesi'.
'amnesi'+'a'='AMNESIA'
(Other definitions for amnesia that I've seen before include "Clinical state of forgetfulness" , "Medical condition" , "memory problem" , "Is a name (anag.)" , "Seaman I found in state of memory loss" .)