Tinker with an instrument (6)
I believe the answer is:
fiddle
I believe this is a double definition.
'tinker' is the first definition.
(I've seen this before)
'an 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 "Steal from one's employer" , "Cheat on (e.g. expenses)" , "Play - cheat" , "tinker" , "Fix" .)