Extravagant queen gets aquatic beast (5)
I believe the answer is:
otter
'aquatic beast' is the definition.
(animal that lives by water)
'extravagant queen' is the wordplay.
'extravagant' becomes 'OTT' (short for over the top).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'ott'+'er'='OTTER'
'gets' is the link.
(Other definitions for otter that I've seen before include "Aquatic carnivore" , "Fish-eating mammal" , "Sleek river mammal" , "Lithe swimmer" , "Mammal, 'Tarka'" .)