The phoney siren goes off outside – nice diversion (9)
I believe the answer is:
insincere
'the phoney' is the definition.
(thesaurus)
'siren goes off outside nice diversion' is the wordplay.
'goes off' is an anagram indicator.
'outside' is an insertion indicator (some letters go outside others).
'diversion' is an anagram indicator.
'siren' anagrammed gives 'insre'.
'nice' is an anagram of 'ince'.
'insre' enclosing 'ince' is 'INSINCERE'.
(Other definitions for insincere that I've seen before include "Less than honest" , "may be laying it on" , "Dishonest" , "displaying hypocrisy" , "fake" .)