Fanciful idea (4)
I believe the answer is:
whim
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fanciful idea</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for whim that I've seen before include "Sudden or capricious idea" , "Urge" , "Fanciful idea" , "Sudden odd desire" , "Caprice, sudden fancy" .)