Night flier (4)
I believe the answer is:
moth
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">night flier</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 moth that I've seen before include "Four-winged insect" , "Nocturnal winged insect" , "Night flier" , "Night flying insect" , "Usually nocturnal insect" .)