Marine museums (7)
I believe the answer is:
aquaria
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">marine museums</span>' is the definition.<br/ ><span class="explanation-format__explanation">('aquarium' can be a synonym of 'marine museum')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for aquaria that I've seen before include "Transparent fish tanks" , "Tanks or buildings for fish" , "display tanks" .)