Tamper with instrument (6)
I believe the answer is:
fiddle
I believe this is a double definition.
'tamper with' is the first definition.
(thesaurus)
'instrument' is the second definition.
(fiddle is a kind of instrument)
(Other definitions for fiddle that I've seen before include "The poor man's violin" , "Swindle with informal violin" , "Toy - string instrument - cheat" , "something stringy?" , "Commit fraud; instrument" .)