"Rainbow" fish (5)
I believe the answer is:
trout
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rainbow fish?</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for trout that I've seen before include "Oily fish" , "Freshwater game fish" , "Prized river fish" , "interfering old woman" , "Tickled fish?" .)