Midday about time? It’s impracticable (3,2)
I believe the answer is:
not on
'impracticable' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'midday about time?' is the wordplay.
'midday' becomes 'noon' (synonyms).
'about' is an insertion indicator.
'time?' becomes 't' (abbreviation).
'noon' enclosing 't' is 'NOT ON'.
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for not on that I've seen before include "Informally, cannot be permitted or tolerated" , "Informally, can't be accepted or tolerated" , "It's unfeasible" , "Modern phrase for unacceptable" , "taboo" .)