Marine mammals (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">marine mammals</span>' is the definition.<br/ ><span class="explanation-format__explanation">(aquatic animals)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for seals that I've seen before include "Marine mammals" , "they're impressive" , "Fastens or closes securely" , "Animals spotted on coast" , "Great swimmers" .)