Observe what water is when warm (6)
I believe the answer is:
notice
'observe' is the definition.
(I know that notice is a more specific form of observe)
'water is when warm' is the wordplay.
I cannot really understand how this works, but
'water' could be 'ice' (ice water is a kind of water) and 'ice' is found within the answer.
The remaining letters 'not' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'what' acts as a link.
Can you help me to learn more?
(Other definitions for notice that I've seen before include "Mark" , "Observe - written announcement" , "This is usually given before quitting job" , "'Perceive, observe (6)'" , "Advance information" .)