Press down (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">press down</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for tamp that I've seen before include "Pack, force down" , "Firmly ram down" , "Pack (eg earth) down firmly" , "Push down tobacco" , "Pack down eg tobacco in a pipe" .)