So about-turn and leave (4)
I believe the answer is:
ergo
'so' is the definition.
('ergo' can be a synonym of 'so')
'about-turn and leave' is the wordplay.
'about' becomes 're' (regarding**).
'turn' shows that the letters should be reversed in order.
'and' means one lot of letters go next to another.
'leave' becomes 'go'.
're' backwards is 'er'.
'er'+'go'='ERGO'
(Other definitions for ergo that I've seen before include "Therefore (Latin)" , "Thus" , "'Cogito, ---- sum'" , "'Therefore, to the old Romans (4)'" , "It follows" .)