Father acting, not son? It’s a façade (8)
I believe the answer is:
frontage
'facade' is the definition.
(synonyms)
'father acting not son? it's a' is the wordplay.
'father' becomes 'fr' (abbreviation as in a priest's title).
'acting' becomes 'on' (I've seen this in other clues).
'not' is a deletion indicator.
'it's' becomes 'tag's' (playground catching game).
'a' becomes 'one'.
'tags'+'one'='tagsone'
'tagsone' with 'son' removed is 'tage'.
'fr'+'on'+'tage'='FRONTAGE'
(Other definitions for frontage that I've seen before include "Facade of a building" , "Measure of property" , "A building's facade" .)