Novel altered by lie, on purpose (12)
I believe the answer is:
deliberately
'on purpose' is the definition.
(synonyms)
'novel altered by lie' is the wordplay.
'novel' indicates an anagram (letters in a new or novel order).
'altered'+'by'+'lie'='alteredbylie'
'alteredbylie' anagrammed gives 'DELIBERATELY'.
(Other definitions for deliberately that I've seen before include "Consciously and intentionally" , "On purpose" , "as intended" .)