Statement lamenting old time, unencoded message format (5,4)
I believe the answer is:
plain text
'message format' is the definition.
The definition suggests a singular noun which matches the answer.
'statement lamenting old time unencoded' is the wordplay.
I cannot really understand how this works, but
'statement' could be 'plaint' (plaint is a kind of statement) and 'plaint' is present in the answer.
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is present in the answer.
'time' could be 't' (abbreviation) and 't' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for plain text that I've seen before include "Just characters" , "easily legible stuff" , "straightforward message" , "easily understandable form" , "en clair" .)