S American instrument mostly without a label on (10)
I believe the answer is:
patagonian
's american' is the definition.
(Patagonia is a region of South America)
'instrument mostly without a label on' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found in the answer.
'label' could be 'tag' (tag is a kind of label) and 'tag' is located in the answer.
'on' is within the answer.
This explanation may well be incorrect...
(Other definitions for patagonian that I've seen before include "From southern Argentina and Chile" , "South American" .)