So three misbehaving are given a warning (9)
I believe the answer is:
therefore
'so' is the definition.
(I've seen this before)
'three misbehaving are given a warning' is the wordplay.
'misbehaving' indicates anagramming the letters.
'are given' says to put letters next to each other.
'a warning' becomes 'fore' (I've seen this before).
'three' anagrammed gives 'there'.
'there'+'fore'='THEREFORE'
(Other definitions for therefore that I've seen before include "Ergo" , "Consequently - for that reason" , "It follows" , "Thus" , "In consequence" .)