Warn of tiny criminal (6)
I believe the answer is:
notify
'warn' is the definition.
(thesaurus)
'of tiny criminal' is the wordplay.
'criminal' indicates anagramming the letters.
'of'+'tiny'='oftiny'
'oftiny' with letters rearranged gives 'NOTIFY'.
(Other definitions for notify that I've seen before include "Declare" , "alert" , "Make known" , "'Advise, tell about (6)'" , "Formally advise" .)