Sir Lancelot 542

[Mayflower3]

Just three got me stumped here -
21a.Dish of duck given to author, having not finished missive(8)O?E?E???
why should it be omelette, is it letter minus r?
16a. Mountain directions taken in by mother(4)?E?A
13d)worries of a soldier on station second to last(10)?G?U???O?S
cant make this out at all!

[sandyRoe]

omelette (r)

[Mamyalynne]

16 Mesa

[sandyRoe]

16 Mesa

[ulysses100]

13d agitations

[Lie-in King]

21a Omelette - 'o' (duck; nil) + 'me' (setter; author) + 'lette(r)'
16a Mesa - 'E(ast) & S(outh)' (in) 'ma'
13d Agitations - 'A GI' + 'tation[s]' (second, to last [position])

[Mayflower3]

Thank you very much - I had Dung Beetle, no wonder!