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' shows that the letters should be reversed in order (in a down clue, the word stands upside down).
'again' becomes 're' (prefix meaning 'again').
'and' is a charade indicator (letters next to each other).
'leave' becomes 'go' (to go is to depart or leave).
're' back-to-front is '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" .)