Warning - the water must be above freezing point (6)
I believe the answer is:
notice
'warning' is the definition.
(I've seen this before)
'the water must be above freezing point' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'water' could be 'ice' (ice water is a kind of water) and 'ice' is present in the answer.
'point' could be 'n' (N is an example) and 'n' is found within the answer.
A single letter 'o' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for notice that I've seen before include "Remark upon; small advertisement" , "Observe; advertisement" , "Formal advice - to quit?" , "Spot - review" , "'Perceive, observe (6)'" .)