Go back again, a month back, to mid-September (6)
I believe the answer is:
recede
'go back' is the definition.
(I've seen this before)
'again a month back to mid-september' is the wordplay.
'again' becomes 're'.
'a month' becomes 'dec' (Dec is an example).
'back' says the letters should be written in reverse.
'to mid' says to take the centre.
'september' becomes 'sep' ('Sep' can be a synonym of 'september').
The middle of 'sep' is 'e'.
'dec' in reverse letter order is 'ced'.
're'+'ced'+'e'='RECEDE'
(Other definitions for recede that I've seen before include "Withdraw" , "Start balding" , "Decline" , "Move backward" , "Become more distant" .)