Protective helmet (7)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">protective helmet</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 hard hat that I've seen before include "Protective headgear" , "Builder's safety wear" , "'nutcase'" , "Headgear for building sites, etc" , "Derby, perhaps" .)