Treat badly when Erato, say, is included (6)
I believe the answer is:
misuse
'treat badly' is the definition.
(I've seen this before)
'erato say is included' is the wordplay.
'erato say' becomes 'muse' (erato is a kind of Muse).
'included' indicates putting letters inside.
'muse' placed around 'is' is 'MISUSE'.
'when' acts as a link.
(Other definitions for misuse that I've seen before include "Ill-treatment" , "bad practice" , "Treat badly, unfairly" , "bad treatment" , "Poor treatment" .)