Foolishly, we have case of tequila in unusually warm weather (4,4)
I believe the answer is:
heat wave
'unusually warm weather' is the definition.
(period of unusual heat)
'foolishly we have case of tequila' is the wordplay.
'foolishly' is an anagram indicator.
'case of' says to hollow out the word (remove centre letters) (outsides of).
'tequila' with its centre taken out is 'ta'.
'we'+'have'+'ta'='wehaveta'
'wehaveta' is an anagram of 'HEAT WAVE'.
'in' is the link.
(Other definitions for heat wave that I've seen before include "Continuous spell of hot weather" , "Long period of hot weather" , "Period of unusually warm weather" , "Prolonged period of very warm weather" , "Spell of very warm weather" .)