Afterthought about one’s church sign (6)
I believe the answer is:
pisces
'sign' is the definition.
(I know that Pisces is a sign)
'afterthought about one's church' is the wordplay.
'afterthought' becomes 'ps' (ps stands for postscript).
'about' is an insertion indicator.
'one's' becomes 'I's' (Roman numeral).
'church' becomes 'CE' (abbreviation for Church of England).
'is'+'ce'='isce'
'ps' placed around 'isce' is 'PISCES'.
(Other definitions for pisces that I've seen before include "Twelfth zodiacal sign" , "The twelfth sign of the zodiac, the fishes" , "Fish star sign" , "stars" , "'Sign of the zodiac, the fish (6)'" .)