Read out, say, writing that's depressingly dull in literary terms (5)
I believe the answer is:
drear
'depressingly dull in literary terms' is the definition.
(literary equivalent to 'dreary')
'read out say writing' is the wordplay.
I cannot really see how this works, but
an anagram of 'read' is 'drea' which is located in the answer.
'writing' could be 'r' (one of the traditional three R's of education) and 'r' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for drear that I've seen before include "miserable" , "Cheerless (literary)" , "Grim" , "Gloomy and cheerless" , "Dismal, dull" .)