Chill, mostly icy, around zero (4)
I believe the answer is:
cool
'chill' is the definition.
(synonyms)
'mostly icy around zero' is the wordplay.
'mostly' means to remove the last letter.
'icy' becomes 'cold' (similar in meaning).
'around' indicates putting letters inside.
'zero' becomes 'o'.
'cold' with its final letter removed is 'col'.
'col' enclosing 'o' is 'COOL'.
(Other definitions for cool that I've seen before include "Unemotional; excellent" , "Level-headed" , "At lowish temperature" , "Excellent; not excited" , "Moderately cold" .)