Swift, rapid (5)
I believe the answer is:
fleet
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">swift</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">rapid</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for fleet that I've seen before include "Fast - naval force" , "cars en masse?" , "Fast; London street, old prison" , "Swift; armada" , "fast is observed" .)