May bring rain when it's terribly cold around mid-January (5)
I believe the answer is:
cloud
'may bring rain when' is the definition.
I can't judge whether this defines the answer.
'terribly cold around mid-january' is the wordplay.
'terribly' indicates an anagram.
'around' means one lot of letters goes inside another.
'mid' says to take the centre (mid- can mean the middle of something).
The centre of 'january' is 'u'.
'cold' is an anagram of 'clod'.
'clod' going around 'u' is 'CLOUD'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for cloud that I've seen before include "Eg, cumulus" , "Become less transparent" , "Mass of airborne water vapour" , "Dull spot" , "Mass of water droplets" .)