Hence, they make about-turn and leave (4)
I believe the answer is:
ergo
'hence' is the definition.
(I've seen this before)
'about-turn and leave' is the wordplay.
'about' becomes 're' (regarding).
'turn' shows that the letters should be reversed in order.
'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'
'they make' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ergo that I've seen before include "for this reason" , "Hence" , "as a consequence" , "Consequently" , "'Cogito, ---- sum'" .)