Stick to one theme on this instrument (4)
I believe the answer is:
harp
I believe this is a double definition.
'stick to one theme' is the first definition.
(I've seen this before)
'this instrument' is the second definition.
(harp is a kind of instrument)
'on' acts as a link.
(Other definitions for harp that I've seen before include "Stick to one theme" , "Large plucked instrument" , "Musical symbol" , "''The ... that once through Tara's halls'' (4)" , "Emblem of Ireland" .)