Writer was angry, maybe about Thackeray hero (9)

'thackeray hero' is the definition.
(character from William Thackeray's novel Pendennis)

'writer was angry maybe about' is the wordplay.
'writer' becomes 'pen' (something which writes).
'was angry maybe' becomes 'sinned' (anger or wrath is a sin).
'about' says the letters should be written in reverse.
'sinned' back-to-front is 'dennis'.
'pen'+'dennis'='PENDENNIS'