Standard of perfection (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">standard of perfection</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 ideal that I've seen before include "unlikely to be bettered?" , "Model of excellence (5)." , "Paradigm" , "fine" , "Nonpareil" .)