Month before one gets the German for crucial game (7)

I believe the answer is:

decider

I'm a little stuck... Click here to teach me more about this clue!

'crucial game' is the definition.
'decider' can be an answer for 'game' (I have seen 'important game' mean 'decider' so perhaps 'game' could also mean 'decider'). I'm unsure of the 'crucial' bit.

'month before one gets the german' is the wordplay.
'month' becomes 'dec' (abbreviation for December).
'before one' becomes 'i' (Roman numeral. I am not sure about the 'before' bit.).
'gets' means one lot of letters go next to another.
'the german' becomes 'der' ('the' in German).
'dec'+'i'+'der'='DECIDER'

'for' is the link.

Can you help me to learn more?

(Other definitions for decider that I've seen before include "Crucial final game" , "Important match" , "Umpire" , "It settles the matter" , "Last game after a draw" .)

I've seen this clue in The Daily Express.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.