Less than (5)
I believe the answer is:
under
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">less than</span>' is the definition.<br/ ><span class="explanation-format__explanation">(eg under a certain number)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for under that I've seen before include "Subject to the authority of" , "In the process of" , "See 16 Down" , "Covered" , "One prepared to take over from leading actor" .)