Intolerable midday temperature inside (3,2)
I believe the answer is:
not on
'intolerable' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'midday temperature inside' is the wordplay.
'midday' becomes 'noon' (synonyms).
'temperature' becomes 't' (abbreviation).
'inside' is an insertion indicator.
'noon' placed around 't' is 'NOT ON'.
Can you help me to learn more?
(Other definitions for not on that I've seen before include "Informally, can't be accepted or tolerated" , "Unacceptable (colloq.)" , "It's unfeasible" , "Colloquially, cannot be tolerated or accepted" , "Improper - impossible" .)