In winter a really warm spell is unusual (4)
I believe the answer is:
rare
'unusual' is the definition.
(rare can mean unusual or odd)
'in winter a really warm' is the wordplay.
'in' says the answer is hidden in the clue.
'RARE' is hidden within 'winter a really warm'.
'spell is' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for rare that I've seen before include "Occurring infrequently" , "Not easily come by like undercooked meat" , "thin on the ground" , "like Ben Jonson, Jack Young thought" , "having few matches?" .)