Warm water to them is a vital necessity (8,4)
I believe the answer is:
tropical fish
'warm water to them is a vital necessity' is the definition.
I can't tell whether this defines the answer.
'warm water to them is a' is the wordplay.
I cannot quite understand how this works, but
'warm' could be 'tropic' (I've seen this in another clue) and 'tropic' is found in the answer.
'is' is present in the answer.
'a' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tropical fish that I've seen before include "We keep them in tanks" , "Hi for plastic (anag) - tank needed?" .)