Month I have to follow is crucial (8)
I believe the answer is:
decisive
'crucial' is the definition.
('decisive' can be similar in meaning to 'crucial')
'month i have to follow is' is the wordplay.
'month' becomes 'dec' (Dec is an example).
'i have' becomes 'ive'.
'to follow' says to put letters next to each other.
'ive' after 'is' is 'isive'.
'dec'+'isive'='DECISIVE'
(Other definitions for decisive that I've seen before include "Settling something conclusively, as battle may be" , "Showing firmness, making mind up without dithering" , "Able to make up one's mind quickly and with firmness" , "Settling an issue quickly" , "Critical" .)