Lecture lad with mice being mistreated (7)
I believe the answer is:
declaim
'lecture' is the definition.
(thesaurus)
'lad with mice being mistreated' is the wordplay.
'with' says to put letters next to each other.
'being mistreated' indicates an anagram.
'lad' put after 'mice' is 'micelad'.
'micelad' with letters rearranged gives 'DECLAIM'.
(Other definitions for declaim that I've seen before include "How orator will" , "Utter rhetorically" , "Orate" , "Harangue" , "Deliver an impassioned speech" .)