Say yes, a second was posted (6)
I believe the answer is:
assent
'say yes' is the definition.
(I've seen this before)
'a second was posted' is the wordplay.
I cannot really see how this works, but
'a' is found in the answer.
'second' could be 's' (common abbreviation) and 's' is present in the answer.
'posted' could be 'sent' ('send' can be a synonym of 'post') and 'sent' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for assent that I've seen before include "Consent or approval" , "Expression of agreement" , "Express approval or agreement" , "Agree - concede" , "Compliance, agreement" .)