Weather mostly terrible includes a very hot spell (4,4)
I believe the answer is:
heat wave
'hot spell' is the definition.
(synonyms)
'weather mostly terrible includes a very' is the wordplay.
'mostly' means to remove the last letter.
'terrible' is an anagram indicator.
'includes' is an insertion indicator.
'very' becomes 'v' (abbreviation).
'weather' with its final letter taken away is 'weathe'.
'weathe' with letters rearranged gives 'heatwe'.
'a'+'v'='av'
'heatwe' enclosing 'av' is 'HEAT WAVE'.
(Other definitions for heat wave that I've seen before include "Spell of very warm weather" , "Period of unusually hot weather" , "Long spell of hot weather" , "Long period of hot weather" , "Spell of unusually hot weather" .)