Formal attire (7,5)
I believe the answer is:
evening dress
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">formal attire</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 evening dress that I've seen before include "Formal dinner wear" , "DJ" , "Formal attire" .)