The Senior Service (4)
I believe the answer is:
navy
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">the senior service</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for navy that I've seen before include "Colour of the fleet" , "Fleet - blue" , "Shade of the marine forces" , "Ships-of-war" , "Senior service" .)