"The treatment," they say crossly, "is included" (7)
I believe the answer is:
therapy
'the treatment' is the definition.
(synonyms)
'they say crossly is included' is the wordplay.
'say crossly' becomes 'rap' (I can't justify this - if you can you should give a lot more credence to this answer).
'is included' indicates putting letters inside.
'they' enclosing 'rap' is 'THERAPY'.
Can you help me to learn more?
(Other definitions for therapy that I've seen before include "will do you good" , "Curative treatment" , "Treatment designed to cure illness or disability" , "Relieving treatment" , "Remedial treatment" .)