First course almost 60 per cent of lunch — dig in! (8)

I believe the answer is:

entrench

'dig in' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for entrench that I've seen before include "Fortify with ditch" , "Establish firmly so that change is difficult" , "Establish firmly in well-defended position" , "Fix firmly or securely" , "Set" .)

I've seen this clue in The Times.

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