Is a wrong number, silly! (7)
I believe the answer is:
asinine
'silly' is the definition.
(I've seen this before)
'is a wrong number' is the wordplay.
'wrong' indicates anagramming the letters.
'number' becomes 'nine' (nine is a number).
'is'+'a'='isa'
'isa' is an anagram of 'asi'.
'asi'+'nine'='ASININE'
(Other definitions for asinine that I've seen before include "Is Annie? (Anag)" , "Senseless" , "'Mindless, vacuous (7)'" , "Is inane to be foolish" , "Stubborn" .)