So stand up again and leave (4)
I believe the answer is:
ergo
'so' is the definition.
(synonyms)
'stand up again and leave' is the wordplay.
'stand up' says the letters should be written in reverse (in a down clue, the word stands upside down).
'again' becomes 're'.
'and' is a charade indicator (letters next to each other).
'leave' becomes 'go'.
're' written backwards gives 'er'.
'er'+'go'='ERGO'
(Other definitions for ergo that I've seen before include "for this reason" , "Hence" , "Therefore - what a goer" , "Therefore (Latin)" , "as a consequence" .)