Big horn (4)
I believe the answer is:
tuba
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">big horn</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tuba that I've seen before include "horn" , "A bass instrument" , "Musical instrument -- 5 (rev)" , "bit of brass" , "One's blown" .)