Main course my speciality! (8)

I believe the answer is:

helmsman

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'main' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for helmsman that I've seen before include "Steersman" , "source of direction" , "One steers a boat" , "Ship's steerer" , "Boat steerer" .)

I've seen this clue in The Sun.

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