Old arbalest could be such an old instrument (9)
I believe the answer is:
astrolabe
'an old instrument' is the definition.
(I've seen this before)
'old arbalest could be' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'could be' indicates an anagram.
'o'+'arbalest'='oarbalest'
'oarbalest' anagrammed gives 'ASTROLABE'.
'such' acts as a link.
(Other definitions for astrolabe that I've seen before include "Navigational device" , "Observational device" , "Navigation aid" , "Old navigation instrument" , "Early astronomical instrument" .)