Missile, briefly (4)
I believe the answer is:
icbm
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">missile briefly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(short for intercontinental ballistic missile; 'briefly' indicates a short form)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for icbm that I've seen before include "Missile" , "Kind of powerful rocket" .)