Thus, therefore (4)
I believe the answer is:
ergo
This clue is a double definition.
'thus' is the first definition.
(I've seen this in another clue)
'therefore' is the second definition.
(I've seen this in another clue)
(Other definitions for ergo that I've seen before include "'Therefore, to the old Romans (4)'" , "Consequently" , "as a consequence" , "'Cogito, ---- sum'" , "for this reason" .)