Smell (unpleasantly) (4)
I believe the answer is:
pong
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">smell unpleasantly</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 pong that I've seen before include "Niff" , "It may make one wrinkle nose" , "In slang, a real stink" , "Ping's regular partner?" , "Horrid smell (informal)" .)