Immature frog (7)
I believe the answer is:
tadpole
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">immature frog</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tadpole that I've seen before include "Frog in the making" , "Potential frog" , "Future frog maybe" , "Pre-frog form" , "young swimmer" .)