It’s expensive to keep writing run-of-the-mill stuff (5)
I believe the answer is:
drear
'it's expensive to keep writing run-of-the-mill stuff' is the definition.
I can't judge whether this defines the answer.
'it's expensive to keep writing' is the wordplay.
'it's expensive' becomes 'dear' (similar in meaning).
'to keep' is an insertion indicator.
'writing' becomes 'r' (one of the traditional three R's of education).
'dear' enclosing 'r' is 'DREAR'.
Can you help me to learn more?
(Other definitions for drear that I've seen before include "Dismal, dull" , "Cheerless (literary)" , "Dismal (literary)" , "miserable" , "Dull and tedious" .)