Alternately sunny/colder - it's relative (5)
I believe the answer is:
uncle
'it's relative' is the definition.
(uncle is a kind of relative)
'alternately sunnycolder' is the wordplay.
'alternately' indicates alternate letters.
The alternating letters of 'sunnycolder' are 'UNCLE'.
(Other definitions for uncle that I've seen before include "older relative" , "Male family member" , "Family relation" , "Dad's brother" , "Pawnbroker's nickname" .)