Brief letter (4)
I believe the answer is:
note
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">brief letter</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 note that I've seen before include "Take this to heed" , "Observe; paper money" , "Put down" , "Short written message" , "Short message; piano key" .)