Writer’s novel reaching all-time low in retrospect (8)
I believe the answer is:
sheridan
'writer's' is the definition.
(I have seen 'Irish writer' mean 'sheridan' so perhaps 'writer' could also mean 'sheridan')
'novel reaching all-time low in retrospect' is the wordplay.
'novel' becomes 'she' (novel by H Rider Haggard).
'reaching' is a charade indicator (letters next to each other).
'all-time low' becomes 'nadir' (I've seen this before).
'in retrospect' says the letters should be written backwards (in retrospect means looking back).
'nadir' written backwards gives 'ridan'.
'she'+'ridan'='SHERIDAN'
(Other definitions for sheridan that I've seen before include "Who scandalised the educational establishment?" , "School for Scandal playwright" , "- Le Fanu, Ir. novelist" , "Creator of Mrs Malaprop in The Rivals" , "general, esteemed by Americans" .)