A foot in a line (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a foot in a line</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for iamb that I've seen before include "Part of a pentameter?" , "Metrical foot of one short, one long syllables" , "Short-long metric foot" , "Short-long metrical foot" , "Metrical unit" .)