What finishes off tern that is after fish sandwich (6)

I believe the answer is:

sarnie

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

I can't explain the remainder of the clue.

(Other definitions for sarnie that I've seen before include "Bread and filling snack (informal)" , "Popular bread-based snack (colloq)" , "Familiar form of snack" , "Sandwich (slang)" , "no classy fare" .)

I've seen this clue in The Times and The Sunday Times.

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