Nothing we're told in letter is permissible (4)
I believe the answer is:
okay
'permissible' is the definition.
(I've seen this in another clue)
'nothing we're told in letter' is the wordplay.
I cannot really see how this works, but
'nothing' could be 'o' (looks like zero - 0) and 'o' is found in the answer.
'letter' could be 'kay' (I have seen 'Eleventh letter ' mean 'kay' so perhaps 'letter' could also mean 'kay') and 'kay' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for okay that I've seen before include "Passable" , "Alright" , "Permissible; fairly good" , "Agree" , "Informally, all right" .)