Envisage about month in retreat, for break away (6)
I believe the answer is:
secede
'for break away' is the definition.
(synonyms)
'envisage about month in retreat' is the wordplay.
'envisage' becomes 'see' (seeing is a kind of envisaging).
'about' means one lot of letters goes inside another.
'month' becomes 'dec' (abbreviation for December).
'in retreat' is a reversal indicator.
'dec' reversed gives 'ced'.
'see' placed around 'ced' is 'SECEDE'.
(Other definitions for secede that I've seen before include "Withdraw from, say organisation" , "Withdraw (from organization)" , "Break away (politically)" , "'Withdraw, break away (6)'" , "leave union" .)