So you take leave at the end of September (4)
I believe the answer is:
ergo
'so' is the definition.
('ergo' can be a synonym of 'so')
'you take leave at the end of september' is the wordplay.
'you take' becomes 'e' (I am not sure about this - if you are sure you should believe this answer much more).
'leave' becomes 'go'.
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'the end of' says to take the final letters.
The final letter of 'september' is 'r'.
'go' put after 'r' is 'rgo'.
'e'+'rgo'='ERGO'
Can you help me to learn more?
(Other definitions for ergo that I've seen before include "Therefore - what a goer" , "Consequently" , "'Therefore, to the old Romans (4)'" , "Hence" , "'Cogito, ---- sum'" .)