Secret agent (3)
I believe the answer is:
spy
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">secret agent</span>' is the definition.<br/ ><span class="explanation-format__explanation">(spy is a kind of secret agent)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for spy that I've seen before include "Catch sight of" , "Glimpse" , "Notice" , "Spot" , "One gathers information surreptitiously" .)