Secret plan (4)
I believe the answer is:
plot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">secret plan</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for plot that I've seen before include "Small area of ground" , "Scheme, story and piece of ground" , "Vegetable garden - scheme" , "garden area" , "Small piece of ground" .)