Constant routine kept up in month: a critical moment (8)
I believe the answer is:
juncture
'a critical moment' is the definition.
(I've seen this before)
'constant routine kept up in month' is the wordplay.
'constant' becomes 'c' (common symbol in maths for an unknown constant).
'routine' becomes 'rut' (being stuck in a rut is being stuck in a routine).
'kept up' says the letters should be written in reverse (in a down clue, letters go up).
'in' means one lot of letters goes inside another.
'month' becomes 'june' (June is an example).
'rut' backwards is 'tur'.
'c'+'tur'='ctur'
'ctur' going inside 'june' is 'JUNCTURE'.
(Other definitions for juncture that I've seen before include "Critical moment" , "Point in time - point where things are joined" , "(Critical) point in time" , "Crisis point when a critical choice must be made" , "A particular point in events or time" .)