Tanks containing, for example, an extremely corrosive mixture (4,5)
I believe the answer is:
aqua regia
'corrosive mixture' is the definition.
(I've seen this before)
'tanks containing for example' is the wordplay.
'tanks' becomes 'aquaria' (aquarium is a kind of tank).
'containing' is an insertion indicator.
'for example' becomes 'e.g.' (short for 'exempli gratia', 'for example' in Latin).
'aquaria' enclosing 'eg' is 'AQUA-REGIA'.
'an extremely' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for aqua regia that I've seen before include "Nitrohydrochloric acid dissolving gold" , "Corrosive mixture" .)