Leant on a fabrication about newborns (8)
I believe the answer is:
neonatal
'about newborns' is the definition.
(I know this)
'leant on a fabrication' is the wordplay.
'fabrication' is an anagram indicator.
'leant'+'on'+'a'='leantona'
'leantona' is an anagram of 'NEONATAL'.
(Other definitions for neonatal that I've seen before include "Of a new-born infant" , "Of or for the newly born" , "Describing newly-born" , "that's been given a new life" , "Relating to an infant during its first month" .)