Send away detectives and teacher (7)
I believe the answer is:
dismiss
'send away' is the definition.
(synonyms)
'detectives and teacher' is the wordplay.
'detectives' becomes 'dis' (Detective Inspectors, DIs).
'and' is a charade indicator (letters next to each other).
'teacher' becomes 'miss' (a female teacher might be addressed 'miss').
'dis'+'miss'='DISMISS'
(Other definitions for dismiss that I've seen before include "P45" , "Sack - cashier" , "Throw out" , "Fire - cashier" , "Cease to consider" .)