Some nice days are cold! (4)
I believe the answer is:
iced
'cold' is the definition.
(similar in meaning)
'some nice days are' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ICED' is hidden within 'nice days are'.
(Other definitions for iced that I've seen before include "Decorated with frosting" , "Core; card; organ" , "Frosted over" , "not at all hot" , "despatched to American underworld" .)