Instrument of torture ultimately found by kingdom (3)
I believe the answer is:
uke
'instrument' is the definition.
(uke is a kind of instrument)
'torture ultimately found by kingdom' is the wordplay.
'ultimately' suggests the final letters.
'found by' is a charade indicator (letters next to each other).
'kingdom' becomes 'UK' (UK is an example).
The final letter of 'torture' is 'e'.
'e' put after 'uk' is 'UKE'.
'of' is the link.
(Other definitions for uke that I've seen before include "Music producer" , "George Formby's instrument (abbr)" , "Notes producer sometimes plucked" .)