An ice cream one is a cold one (4)
I believe the answer is:
cone
'an ice cream' is the definition.
(cone can refer to an ice cream cone)
'cold one' is the wordplay.
'cold' becomes 'C' (eg on taps).
'c'+'one'='CONE'
'one is a' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for cone that I've seen before include "Hollow object associated with ice cream and motorways" , "Figure (for traffic control?)" , "Tapering shape" , "Portable marker for lines on road" , "Road furniture!" .)