Opening course for Aintree official? (7)

I believe the answer is:

starter

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'official?' is the definition.
(starter is a kind of official)

'opening course for' is the wordplay.
'opening' becomes 'start' (I've seen this in other clues).
'course for' becomes 'er' (I can't explain this - if you can you should give a lot more credence to this answer).
'start'+'er'='STARTER'

'aintree' is the link.
I am not very happy about this link. It may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for starter that I've seen before include "Hors d'oeuvre" , "First course of meal" , "cooking served first?" , "No substitute" , "First course for beginner" .)

I've seen this clue in The Times.

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