Letter from Greece
I believe the answer is:
psi
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">letter from greece</span>' is the definition.<br/ ><span class="explanation-format__explanation">(letter from the Greek alphabet)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for psi that I've seen before include "foreign letter" , "Greek letter; pounds per square inch (abbrev.)" , "Pounds per sq. in. (abbr.)" , "Greek character" , "Greek letter; pressure unit" .)