Tinker with instrument (6)
I believe the answer is:
fiddle
This clue is a double definition.
'tinker' is the first definition.
(I know that tinker can be written as fiddle)
'instrument' is the second definition.
(fiddle is a kind of instrument)
'with' acts as a link.
(Other definitions for fiddle that I've seen before include "something stringy?" , "Commit fraud; instrument" , "tinker" , "Fix" , "Awkward task" .)