Written in memo, a thoughtless promise (4)
I believe the answer is:
oath
'promise' is the definition.
(thesaurus)
'written in memo a thoughtless' is the wordplay.
'written in' indicates the answer is hidden within the clue.
'OATH' is hidden within 'memo a thoughtless'.
(Other definitions for oath that I've seen before include "Profane or obscene expression" , "Sworn declaration" , "It could be ruddy when minced" , "Solemn promise to tell the truth or be loyal" , "Strong language" .)