Very keen on (4)
I believe the answer is:
into
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">very keen on</span>' is the definition.<br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for into that I've seen before include "Two -- six is three" , "A fan of" , "Fascinated by" , "ignition regularly missing" , "Knowledgeable about" .)