Tanks having water and fresh air (7)
I believe the answer is:
aquaria
'tanks' is the definition.
(aquarium is a kind of tank)
'water and fresh air' is the wordplay.
'water' becomes 'aqua' (I've seen this in another clue).
'and' says to put letters next to each other.
'fresh' indicates anagramming the letters.
'air' with letters rearranged gives 'ria'.
'aqua'+'ria'='AQUARIA'
'having' acts as a link.
(Other definitions for aquaria that I've seen before include "Transparent fish tanks" , "display tanks" , "Tanks or buildings for fish" .)