It's a thought (4)
I believe the answer is:
idea
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">it's a thought</span>' is the definition.<br/ ><span class="explanation-format__explanation">(idea is a kind of thought)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for idea that I've seen before include "Purpose" , "Flash of inspiration?" , "Thought or notion (4)" , "Thought; concept" , "Content of cognition" .)