Reversible instrument (4)
I believe the answer is:
deed
I believe this clue is a double definition.
'reversible' is the first definition.
'instrument' is the second definition.
(deed is a kind of instrument)
(Other definitions for deed that I've seen before include "Document showing ownership" , "Legal document for transfer of property" , "What's done" , "documentary evidence" , "Feat" .)