Father is happening to adopt one amendment (8)
I believe the answer is:
revision
'amendment' is the definition.
(I've seen this before)
'father is happening to adopt one' is the wordplay.
'father' becomes 'rev' (I've seen this in other clues).
'happening' becomes 'on'.
'to adopt' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'is'+'on'='ison'
'ison' enclosing 'i' is 'ision'.
'rev'+'ision'='REVISION'
(Other definitions for revision that I've seen before include "Preparation for exam" , "Improving change" , "change for the better" , "Action of amending and improving" , "requirement before exam" .)