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