Crossword Genius

Finish one's letters? One may use fingers as a guide (8)


I believe the answer is:

'one may use fingers as a guide' is the definition.
(fingers can mean the angled parts of a signpost)

'finish one's letters?' is the wordplay.
'finish' becomes 'sign' (as in to finish a letter by signing it).
'one's letters?' becomes 'post' (someone's mail).

(Other definitions for signpost that I've seen before include "Clue", "Guide, indicator", "this will give you directions", "Direction indicator", "director".)

I've seen this clue in The Guardian.