English Catholic overwhelmed by the writer’s compassion (5)
I believe the answer is:
mercy
'compassion' is the definition.
(I know that mercy is a type of compassion)
'english catholic overwhelmed by the writer's' is the wordplay.
'english' becomes 'e' (abbreviation).
'catholic' becomes 'rc' (Roman Catholic).
'overwhelmed by' means one lot of letters goes inside another (inserted letters get absorbed or overwhelmed).
'the writer's' becomes 'my' (belonging to the person who set the crossword).
'e'+'rc'='erc'
'erc' put inside 'my' is 'MERCY'.
(Other definitions for mercy that I've seen before include "'The quality of ... is not strained' (The Merchant of Venice)" , "Its quality is not strained, Portia said" , "Forbearance" , "Shakespeare 'The quality of . . . . . is not strained'" , "Compassion or forgiveness" .)