Mark Field 'visiting' Truss and unloading: I'm outraged! (5)

I believe the answer is:

tilde

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'mark' is the definition.
(tilde is a kind of mark)

I don't understand the remainder of the clue.

(Other definitions for tilde that I've seen before include "Augmentation of text" , "Mark placed over N in Spanish" , "Squiggly mark" , "Accent (used over n) in Spanish and Portuguese" , "Sign on Spanish character" .)

I've seen this clue in The Independent.

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