Phoney siren going off outside - nice diversion (9)
I believe the answer is:
insincere
'phoney' is the definition.
(thesaurus)
'siren going off outside nice diversion' is the wordplay.
'going off' indicates an anagram.
'outside' is an insertion indicator.
'diversion' is an anagram indicator.
'siren' anagrammed gives 'insre'.
'nice' is an anagram of 'ince'.
'insre' going around 'ince' is 'INSINCERE'.
(Other definitions for insincere that I've seen before include "may be laying it on" , "fake" , "False, hypocritical" , "Dissembling" , "Untruthful" .)