So is about to turn and leave (4)
I believe the answer is:
ergo
'so' is the definition.
('ergo' can be a synonym of 'so')
'about to turn and leave' is the wordplay.
'about' becomes 're' (regarding**).
'to turn' says the letters should be written backwards.
'and' means one lot of letters go next to another.
'leave' becomes 'go'.
're' written backwards gives 'er'.
'er'+'go'='ERGO'
'is' acts as a link.
(Other definitions for ergo that I've seen before include "so philosopher wrote in Latin" , "for this reason" , "Consequently" , "It follows" , "Therefore - what a goer" .)