Childish lie with pure fabrication (7)
I believe the answer is:
puerile
'childish' is the definition.
(thesaurus)
'lie with pure fabrication' is the wordplay.
'with' says to put letters next to each other.
'fabrication' is an anagram indicator.
'lie' after 'pure' is 'purelie'.
'purelie' anagrammed gives 'PUERILE'.
(Other definitions for puerile that I've seen before include "Babyish" , "Childishly stupid" , "Childish, immature" , "Infantile" , "Childishly silly" .)