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" .)