Infiltrator (4)
I believe the answer is:
mole
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">infiltrator</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for mole that I've seen before include "Spy - one that burrows" , "A spy within an organisation" , "Small congenital skin spot" , "Subterranean mammal" , "spy a > breakwater" .)