Greek H (3)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">greek h</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for eta that I've seen before include "Something like 10 [RHO]" , "Letter of Greek alphabet" , "Letter from abroad" , "giving expected termination of journey" , "Member of Greek set a" .)