Aquarium accessory (7)
I believe the answer is:
aerator
'aquarium accessory' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aerator that I've seen before include "It provides the fizz" , "Device for pumping in air" , "One may emit much gas" , "Device such as used in tropical fish tanks" , "Maybe fork" .)