Forgive old American in church (6)
I believe the answer is:
excuse
'forgive' is the definition.
(excusing is a kind of forgiving)
'old american in church' is the wordplay.
'old' becomes 'ex'.
'american' becomes 'us'.
'in' means one lot of letters goes inside another.
'church' becomes 'ce' (abbreviation for Church of England).
'us' put inside 'ce' is 'cuse'.
'ex'+'cuse'='EXCUSE'
(Other definitions for excuse that I've seen before include "Make apologies for" , "Upstanding community members" , "A self-justification" , "Free - apology" , "Justifying explanation" .)