Observe the water has now thawed? (6)
I believe the answer is:
notice
'observe' is the definition.
(I know that observe can be written as notice)
'water has now thawed?' is the wordplay.
'water' becomes 'ice' (ice water is a kind of water).
'has' says to put letters next to each other.
'now thawed?' becomes 'not' (I can't justify this - if you can you should give a lot more credence to this answer).
'ice' put after 'not' is 'NOTICE'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for notice that I've seen before include "Advance warning of job resignation" , "Bulletin" , "'Observe, discern (6)'" , "Become aware of; review" , "Observe the placard or sign" .)