With no preparation, face the advancing weather change (4,5)
I believe the answer is:
cold front
'with no preparation' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'face the advancing weather change' is the wordplay.
I cannot really see how this works, but
'face' could be 'front' (fronting is a kind of facing) and 'front' is found in the answer.
'weather' could be 'cold' (cold weather is a kind of weather) and 'cold' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cold front that I've seen before include "Meteorological boundary" , "that might bring on a chill" , "which might change the weather" , "freeze?" , "sometimes a feature of weather forecast" .)