Permission unavailable? You don't say! (5,3)
I believe the answer is:
leave out
'you don't say' is the definition.
I can't judge whether this definition defines the answer.
'permission unavailable?' is the wordplay.
'permission' becomes 'leave' (leave is permission or authorisation).
'unavailable?' becomes 'out' (I've seen this in another clue).
'leave'+'out'='LEAVE OUT'
Can you help me to learn more?
(Other definitions for leave out that I've seen before include "Omit or exclude" , "Forget" , "Omit, overlook" , "not put away" , "Drop" .)