Aquarium acquisition (5)
I believe the answer is:
tetra
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">aquarium acquisition</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tetra that I've seen before include "Denoting four (prefix)" , "rarely seen flower" , "This indicates a group of four" , "Brightly coloured aquarium fish" , "Swimmer" .)