Regret second post when over (7)
I believe the answer is:
sadness
'regret' is the definition.
(thesaurus)
'second post when over' is the wordplay.
'second' becomes 's' (common abbreviation).
'post' becomes 'send' ('send' can be a synonym of 'post'**).
'when' becomes 'as'.
'over' is a reversal indicator.
's'+'send'+'as'='ssendas'
'ssendas' reversed gives 'SADNESS'.
(Other definitions for sadness that I've seen before include "Sorrowfulness" , "Emotion experienced when unhappy" , "gravity" , "no joy there" , "distress" .)