Agrees one posted the twit on guard (7)
I believe the answer is:
assents
'agrees one' is the definition.
'assents' can be an answer for 'agrees' (assenting is a kind of agreeing). I'm not sure about the 'one' bit.
'posted the twit on guard' is the wordplay.
I cannot really understand how this works, but
'posted' could be 'sent' ('send' can be a synonym of 'post') and 'sent' is present in the answer.
'twit' could be 'ass' (I've seen this in another clue) and 'ass' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for assents that I've seen before include "Gives nod" , "Expresses agreement" , "Accedes, acquiesces" , "Says yes" , "Agrees, complies" .)