Fall behind (3)
I believe the answer is:
lag
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fall behind</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for lag that I've seen before include "An old one is a convict" , "Wrap water-pipes; fall behine" , "Convict (slang)" , "Wrap to protect from frost" , "Drop behind" .)